This is the 363rd most frequent Czech word.
deseti
"Deseti" in Czech means "of ten" or "ten" in the locative, dative, or genitive case.
The word 'deseti' is used here in the genitive case to specify the quantity of the noun 'nejlepších knih' (best books).
Mám seznam deseti nejlepších knih.
I have a list of the ten best books.
In this sentence, 'deseti' is in the dative and describes the people ('lidem') who benefited from the action.
Deseti lidem pomohl dar organizace.
The organization's donation helped ten people.
Although 'deset' is used here instead of 'deseti', this demonstrates a different case usage (genitive after preposition).