Imagine that in one day you can either mow 12 lawns or knit 3 sweaters. Your brother, on the other hand, can either mow 10 lawns or knit 2 sweaters in the same time. You are objectively better at both tasks, so you have absolute advantage in both, but who has comparative advantage?
For you, the resources (let's call them R) required to mow 12 lawns (L) is equal to the resources required to knit 3 sweaters (S). Since 12L = R and 3S = R then 12L = 3S, L = 0.25S and S = 4L.
For your brother, 10L = R and 2S = R, so 10L = 2S, L = 0.2S, and S = 5L
These equalities are the opportunity cost of each action. In order to mow one lawn, you need to give up enough resources to knit 0.25 sweaters. In order for your brother to knit one sweater, he needs to give up enough resources to mow 5 lawns. Comparative advantage goes to whichever side has the lowest opportunity cost. Since your opportunity cost for producing a sweater is 4 lawns (less than your brother's 5), you have comparative advantage in kniting sweaters. Since your brother's opportunity cost for mowing lawns is 0.2 sweaters (less than your 0.25), he has the comparative advantage in mowing lawns.