A contraction is two words spoken or written as one shorter word, with an apostrophe standing in for the letters that were dropped.
In speech, someone says: I don't know where the keys are. That's do not squeezed into don't; the o of not is replaced by an apostrophe. Other not forms work the same way, like isn't for is not.
In a text message, a friend might write: I'm on my way! Here I am becomes I'm, with the apostrophe marking the dropped a. Pronouns and verbs often pair like this.
The same friend could just as easily type I can't talk now, where cannot shrinks to can't. The meaning is identical; only the letters left out change.
These short forms suit conversation and casual writing. In a formal report you would more likely write do not in full, though a light contraction is acceptable when it keeps the tone natural. The words mean the same either way; only the register changes.
I'd've come if you'd called works fine in conversation, but I would have come reads better in a formal letter. This contraction squeezes three words into one, so it belongs firmly to speech rather than anything written for a formal audience.
One apostrophe trap is worth naming. In a contraction the apostrophe marks missing letters, but in a possessive like the dog's it marks ownership. So The dog's here (short for the dog is) and the dog's tail (belonging to the dog) look identical yet work differently.