Contractions are a very important part of English pronunciation. A contraction is the merging and shortening of two words to make a new word. A letter is removed and replaced with an apostrophe (‘).
This new word means exactly the same as the two separate words. The only difference is that they are easier and faster to pronounce. We use them all the time in informal situations.
They are perfectly acceptable to use (even in formal situations) and handy to know.
There are quite a lot of them but beware as some contractions have multiple meanings. Study the following table:
If you love learning English slang and want to be down with the kids, check out this very VERY informal list of contractions.