A welder needs to know the meaning of polarity and understand how it affects the welding process. Typically, electrode-positive (reversed polarity) welding results in deeper penetration. Electrode-negative (straight polarity) welding results in faster melt-off of the electrode, and therefore a faster deposition rate. Deposition rate refers to the amount of filler metal melted into the weld joint.

On the other hand, AC welding is often used with low-cost, entry-level machinery, making it a good choice for welding training. Many welders prefer it in conditions where the arc can blow side-to-side.

Understanding the Different Types of Polarity

There are three main types of polarity: direct current straight polarity, direct current reverse polarity and alternating current polarity.

Direct Current Straight Polarity

Direct current straight polarity welding happens when the plates are positive and the electrode is negative. This causes the electrons to go from the electrode tip to the base plates.

It’s generally considered that two-thirds (66%) of the entire arc heat is generated at the electrode, whereas only one-third (33%) of the heat is generated at the base plate. As a result, the electrode melts down quickly and the metal deposition rate increases (for consumable electrodes only).

On the other hand, base plates tend to not fuse properly due to a lack of sufficient heat. Therefore various defects arise, such as insufficient fusion, lack of penetration and high reinforcement. Weld reinforcement is a term used to describe metal that is needed to fill a joint.

Direct Current Reverse Polarity Welding (DC Reverse Polarity)

When the electrode is positive and the plates are negative, this results in direct current reverse polarity. The electrons switch directions and go from the base plates to the electrode. Consequently, more heat generates at the base plate as compared with DC straight polarity.

This type of welding is less likely to cause inclusion defects (nonmetallic particles trapped in the weld metal or at the weld interface) due to its arc cleaning action. It makes for faster welding and performs better for welding thin pieces of material. It’s commonly chosen for joining metals like copper, which has a low melting point.

The potential downside to this type of welding is that it has a shorter electrode life. If the speed isn’t set correctly, there is a high level of reinforcement needed. While it works great for thinner materials, this method may be ineffective for joining thick plates with higher melting points.

Alternating Current Polarity

If an AC current is supplied by the power source, reverse and straight polarity will take place one after the other. In half the cycle, the base plates will be positive and the electrode will be negative. In the other half, the electrode will be positive and the base plates will be negative.

Depending on frequency of supply, this cycle repeats 50 to 60 times per second. Some power sources also provide provisions, which can alter frequency.

AC polarity has attributes of straight and reverse polarity, since both are occurring in the same cycle. It is effective to use with most electrode types and is suitable for many different plate thicknesses, making it a great all-around choice

you could find more detailed explanation in the link below

https://weldguru.com/polarity-in-welding/

Welding arc has three distinct portion defined as anode and cathode regions and arc column. This is clearly observed if the voltage drop is measured across the arc. The portions close to anode and cathode regions have large gradients meaning the heat generated close to anode and cathode is high. The gradient is small in column region but it is spread over the whole length of the arc. The heat is generated at anode and cathode due to impact of electron and positive ions there. The exact amount depends upon the type of arc gases and evaporated metal etc. Nevertheless, more heat generates at anode. The simplistic assumption is that 2/3rd heat generates at anode and 1/3rd at cathode. This explains the different amount of heat generation at electrode/work-piece depending on the polarity. The arc is brighter (high temperature) in TIG welding due to type of the gas (pure Argon or Helium) present there. It produces more ultra-violet radiation and needs better protection of the eyes, requiring different type of glass in the hood. More practical aspect of this phenomena is explained in the other answer by Krishna Reddy, so I am not dealing with that.