newbie question

It allows more gas exchange, in some cases, and also allows more evaporative cooling. I run tanks with and without covers. Either way can work, but a cover can cause problems in some setups. It's easier just to leave the top open.
But do cover it with something like nylon screen or eggcrate. As was pointed out, some fish are acrobats.
Or a tall canopy. I have a tall canopy, but it had an open area on the back and a clown and swallowtail angel managed to jump out through there before I got some eggcrate and used that on the back of the canopy to cover the opening.
And (I seem to be appending other replies):D. If you have a eel cover with nylon screen every opening in the tank. They don't jump but they really can slither. ;)
i think what they are referring to is the plastic grid used on ceiling florescent light fixtures sold at do it yourself home supply stores where they sell lighting fixtures. the grid has 1/2 inch openings.
Aren't most canopies open in the back that would not be sufficient for cooling and gas evaporation
A canopy that's open in back should be fine, if there's enough space behind the tank for reasonable air flow. That shouldn't be much of a restriction.