what difference between plural s and third person s?

Answer 1

huge difference. The same form does not have to carry the same meaning in a language!

Answer 2

Plural s is used at the end of regular nouns. It tells you that the reference is to two or more items.

Example: I have one dog but she has two dogs.

Third person s is used at the end of regular verbs in the present simple tense. It is the proper form to use with "he", "she", or "it" in that verb tense.

Example: I walk to the library every day and he walks with me.