Determiners are used to identify things. To take the dictionary definition, they are modifying words that determine the kind of reference a noun or noun group has.
Determiners are different to pronouns in that a determiner is always followed by a noun. Personal pronouns ( I , you , he , etc.) and possessive pronouns (mine, yours, his, etc.) don't act as determiners.
They are used in front of nouns to indicate whether you are referring to something specific or something of a particular type.
The definite and indefinite articles a/an/the are all determiners.
You use a specific determiner when people know exactly which thing(s) or person/people you are talking about.
"The dog barked at the boy."
"These apples are rotten."
"Their bus was late."
"Have you seen my keys?"
You use general determiners to talk about how much stuff or how many people or things you are talking about.
More general determiners are quantifiers:
"Have you got any English books I could borrow?"
"There is enough food to feed everyone."
"I don't teach online every day."
Numbers act as determiners too, they show how many things or people there are: 1, 2, 3...
"I teach online for 3 days a week."
Either and neither are used in sentences concerning a possible choice between two items.
Either can mean one or the other (of two) or each of two.
I've got tea and coffee, so you can have either. (One or the other)
The room has a door at either end. (Both)
Neither means not the first one and not the second one.
Neither of the students were listening.