A square is a figure with four equal sides and four right angles. It's a popular shape for windows and record albums, among many other things.
Square has many different metaphorical uses. Old towns had square open areas for meetings, so now "town square" means meeting place, even if it's not shaped exactly like a square. If you settle your debts, you square them, and if someone calls you a square, they mean you're boring, rule-abiding, and lame. In math, a square is the product of something multiplied by itself. If you square your plans with someone, you line them up.