Lee is right - ain't = is not. However, ain't is used very informally and is not really considered "good English" nowadays. I would not really advocate its use but you will hear it (especially in music and films) so it is useful to know!

it's especially bad when it's used in the following form: It ain't no good literal translation: it is not no good Now, this sentence has 2x a denial. This is used to put more focus on the denial, but is grammatically incorrect, afaik. This is however used widely in songs and on the streets for decades already. Don't write it in your exams tho :D It's better to just say: It's not good instead of It ain't no good or There is no mountain high enough instead of There ain't no mountain high enough

It is a very informal abbreviation for "is not". E.g. "It ain't right" = "It is not right". The better way to abbreviate "is not" is "isn't". E.g. "It isn't right".