Hi Steve. 

Ton, ta, tes and son, sa, ses are called possessive adjectives and adjectives modify nouns so they must agree in gender and number with the noun they’re modifying. Ton, ta, tes (your) are used when the subject pronoun (je, tu, il/elle, nous, vous, ils/elles) is tu, and son, sa, ses (his or her) are used when the subject pronoun is il or elle. Ton and son are the masculine adjectives used before masculine nouns or feminine nouns that begin with a vowel or mute h. Ta and sa are the feminine ones and tes and ses are the plural adjectives (both genders). This can be a little confusing, at least it was for me at first. 

For example: Elle a son livre. = She has her book.

The subject pronoun is “elle” which obviously, is feminine but “son” is used because it modifies “livre” which is masculine. If the noun “livre” was plural,” ie., “livres” then ses would be used.

Tu aimes ta voiture?  Oui, j’adore ma voiture!  = 

You like your car? Yes, I love my car! 

I hope this helps. 

son, sa, and ses can mean either his/her + the noun that that person owns. - Her car - sa voiture or his car - sa voiture. The son is for masculine nouns, sa for feminine nouns and ses for plural nouns- her cats - ses chats or his mother - sa mère - because the mother is feminine. 

Ton, ta, and tes are used for "your" - and the same rules apply as above - your car - ta voiture, your uncle - ton oncle, your cats  - tes chats