These are used to express that the person receiving the action is the subject. These only are declined by case, not gender or number. There is no nominative form for these.
In English, the word się translates to yourself, myself, themselves, etc.
Sometimes, the reflexive particle się can have the same meaning as the reflexive pronouns, so there is a rule as to when each form should be used.
Się is used when the reflexive is detached from the verb.
(I washed myself)
The reflexive pronouns are used when the reflexive is detached form the verb.
Umyłem go, a potem siebie.
(I washed him, then myself)