When we count, we answer “how many?” — one, two, three. Numbers used that way are called cardinal numbers: they tell us the amount.

Ordinal numbers answer a different question: “which one, in order?” They describe a thing's position in a line — who is first, second, third, and so on. We write them with a number and a little ending: 1\text{st}, 2\text{nd}, 3\text{rd}, 4\text{th}, 5\text{th}…

Notice the first three are special — first, second, third — and from fourth onward they simply end in -th. Watch as a row of friends lines up: each one gets its place in the order, and we point out one position along the way.

So the two ideas work together: cardinal says how many are in the line, and ordinal says where each one stands. In a line of 5 children there are five of them (cardinal), and the one at the front is the 1\text{st} (ordinal).