When I hover over the words, I can see that 加 is add while 油 is oil!
Here's some extra knowledge about the two characters if you are interested:
加 is made of 力 and 口. 力 means "power/ strength" while 口 is mouth as you may know. The original meaning of 加 is the exaggeration of speech, and you can guess that from the constituents - "to use strength when your mouth is open". It acquired the meaning of "increase" because when you exaggerate something, then the nature of that thing may be boasted. Hence, the word also refers to add, as add is similar to increase.
油 is made up of the 3 dots on the left which represents water, and 由 on the right. 由 in ancient hieroglyphics was actually a drop of liquid falling onto a bowl (try to imagine), which people later referred to as a bowl with oil. As the meaning of oil transformed, people added 3 dots on the left to show that oil is a liquid.
I've seen 加油 translated in movie subtitles as "Let's go!" "You can do it!" and "Work hard to achieve success!" (as near as I can remember). It was always in the context of encouragement and cheering people on, whether for sports or tests or job interviews. Does anyone have more insight on it's usage or connotations?