I assume the 'the source code' has some bugs or errors that make the grammar of the line indistinguishable, that is the interpreter does not recongnize it or disinguish/identify it.

'negligible', can be said of somethng so small, that is unimportant (almost not to be taken into consideration as important)

Hope it helps 

Hello Amal,

Answers can vary depending on the context but if you speak about probabilistic encryption (which looks like the topic of your other questions), you design a game using your crypto primitive where the adversary has to differentiate the message you ciphered with a junk one. The game should be designed such as the adversary has not more than 50% chance to succeed which is equivalent to a random guess. Then you prove your security by repeating the game as many time as you wish to obtain (1/2^n)% chances of success. 

In this context, indistinguishability means you can't differentiate the real ciphertext with the junk one.

Cheers,

Maxime