Just like Katharine has just explained you, people are totally lazy sometimes and they like to make the words much shorter. For example:
1. Gimme - Give me (for example: "gimme a kiss" instead of "give me a kiss" )
2. Gotta go - I gotta go (For example: "I have to go home" instead of "I have to go home" )
3. CYA - See you (For example: " Cya tomorrow " instead of " See you tomorrow)
I hope I made it claer now to you. If you have any futher questions, feel free to ask me.