Usually,chinese people use this term when one person encourage another person.The meaning of 油 is petrol while 加 is a verb and means add into .To add petrol into one person is just like to put energy into one's body. Nowadays, many chinese people regard it as a phrase like Good Luck.

It means "Come on" or "Keep fighting".If we say it to others, it is used to encourage someone. Sometimes we write it at the end of the sentences to encourage others to show our good wishes for them. If we say “我会加油”，here means "I will try my best."

it means come on

It means encourage oneself.