It is the ability of the Central Bank to influence the banks & financial institutions to pass on the monetary policy movements to the end users or public.

It is the process / mechanism through which the impact of Central Bank's monetary policy decisions are passed on via channels of banks & financial markets, to influence the economy wide factors like general price level & interest rates, national income, and the exchange rate.

The channels which transmit the impact of monetary adjustments to the real economy variables like GDP, unemployment, etc. 

Just as all my friends above said, channels of transmission are the means by which a monetary policy tool reach some variables.

Usually, there are two monetary policy tools, interest rate and regulation. Let us take the interest rate, as it is the most used instrument. 

When the Central Bank changes its interest rate, it changes liquidity at money market and the agents reorder their operations. This remaking operations are the channels of transmission of monetary policy action. Commonly, there are five channels of transmission from the interest rate to aggregate demand: credit channel, portfolio channel, expectations channel, exchange rate channel and wealth channel. The importance of each one depends on the country’s financial system, because it is the spare from which the impacts of monetary polecat action emerges. 

Regulation is more straightforward as it regulates how an agent operates. For example, compulsory reserves are regulation. It has no flexibility and usually it structures how the financial markets operate.

All the best.