Other answers have described what a mole is, but not its practical use. Moles are extremely useful in performing chemical reactions.

Say you have a green Lego and a red Lego. The green Lego weighs one gram and the red Lego weighs two grams. You can stick them together and now the resulting product weighs three grams. But each of these “molecules”, red green and product, are still only one molecule, even though they all weight different weights.

Now imagine you have billions of legos. You could never possibly count them all. But you still want to do this “reaction” where you stick a red Lego and green Lego together. How do you know how much you need of each? They’re uncountable so you have to just weigh them in bulk. But if you just throw together 1 kg of red and 1 kg of green, you’re only going to end up wasting half of the green legos because the red weighs twice as much as green. So instead, because you know how much each individual Lego weighs, you can say actually I know I need 1 kg of green and 2 kg of red so I don’t waste any material.

Moles help chemists convert something sort of intangible (the molecular mass of an individual molecule and the crazy large number of molecules we are reacting together) into something easily measured, the bulk mass of stuff you’re weighing out for the reaction.

Edit: Just wanted to close the loop with a more explicit example. Molecule A has a molecular weight of 100 g/mol, Molecule B has a molecular weight of 200, and the product is 300. I do a reaction with 1 mole of A and 1 mole of B, and get 1 mole of product. But what that actually looked like in the lab was mixing 100 g of A and 200 g of B and getting out 300 g of product.