What in the world is "income" mean?
its located top right corner while playing game.

It means how much gold you get in XX secs

like 10 Gold in 10 secs

or 1000 gold in 10 secs

depends on game

Its been a while since I played melee, but I'm pretty sure he's talking about the number of units you own. Low/Mid/High income which affects how many resources you gain per hit. I don't remember the numbers exactly, but low income (under 40?), you receive full resources, 10 per. At Mid income, it becomes 7, and at low, it becomes 4? Maybe 3? Pretty sure 4, but someone who plays should be able to clarify more.

50 population and under you get full income (10 per gold sack). 51-80 population you get medium income (7 gold per sack) and 81-100 you get low income (4 gold per sack).

Thats called "Upkeep" though