1to add oil; to top up with gas; to refuel
2to accelerate; to step on the gas
3(fig.) to make an extra effort; to cheer sb on
Examples
suī rán虽然míng tiān明天hǎo xiàng好像huì会xià yǔ下雨，dàn但wǒ我huì会jiā yóu加油de的。
虽然明天好像会下雨，但我会加油的。
It looks like rain tomorrow, but I'll try my best.
考试加油！
Good luck on your test.
加油！
Go!