Here's how it works. For every crossword line on the board, you're given a set of math equations with the numbers missing. There's one total number for every puzzle (say, 6), and then for each equation, you need to figure out how another set of numbers fits in to complete the clue and make the equation equal to the total number. In other words, given the clue (*+*) x (*-*), and the answer of 6, you'd eventually work out that the numbers should be (1+2) x (5-3), and then put 1, 2, 5, and 3 in each spot on the answer. But that answer line has to match up to any that it intersects with, so you may need to switch the numbers around -- (2+1) instead of (1+2) -- for it all to work.
It's fiendishly clever, and it really puts your brain to work, especially if (like me) you're not that great at casual math. But there are a ton of puzzles to work through, and then you can even buy a puzzle pack via in-app purchase for more. Especially at the current price of free, Quaso is a mathematical brain teaser that you shouldn't miss.