Ich habe drei Bücher.
English: I have three books.
In German, 'Zahlen' are numbers (like 1, 2, 3), and 'Ordnungszahlen' are ordinal numbers (like first, second, third). They are used to count things and to show order or position.
Use 'Zahlen' to count objects or people, say your age, or talk about quantities. Use 'Ordnungszahlen' to describe the order of things, like dates, rankings, or positions (for example, the first floor, the third day).
Ich habe drei Bücher.
English: I have three books.
Heute ist der erste Mai.
English: Today is the first of May.
Mein Geburtstag ist am zwölften Januar.
English: My birthday is on the twelfth of January.
Er kommt als zweiter ins Ziel.
English: He finishes second.