Leur et ses

 "Leur" and "ses" are both possessive adjectives in French, but they are used to indicate possession differently:

Leur:

Meaning: "Leur" translates to "their" in English.

Usage: "Leur" is used to indicate possession by a group of people or objects (third person plural).

Example: "C'est leur voiture." (It's their car.) 

Here, "leur" agrees with the plural noun "voiture" (car) and indicates that the car belongs to a group of people.

Ses:

Meaning: "Ses" also translates to "their" in English.

Usage: "Ses" is used to indicate possession by a singular noun (third person singular).

Example: "Elle a perdu ses clés." (She lost her keys.) 

In this sentence, "ses" agrees with the plural noun "clés" (keys) and indicates that the keys belong to a singular person.

In summary, "leur" is used with plural nouns to indicate possession by a group of people or objects, while "ses" is used with singular nouns to indicate possession by a singular person. 

Both agree in gender and number with the noun they modify.