Uses of the Spanish pronoun 'se’

1.As a reflexive pronoun

This is its most common use. Se is used as the reflexive pronoun for third-person uses
e.g. Pedro se peina. (Pedro comb his hair)
e.g. Miguel se levanta temprano (Miguel get up early)

2.As the equivalent of the English passive óbice

as a way of indicating that some sort of action is done without indicating who or what performed the action.
e.g. Se vende (to be sold)
e.g Se alquila (to be rented)
e.g. El coche se vendió (the car was sold)

Spanish does have a true passive voice corresponding to the English one: El coche fue vendido ("The car was sold")

3.As a substitute for le or les

When the indirect-object pronoun le or les is immediately followed by another pronoun that begins with an l, the le or les is changed to se, apparently as a way to avoid having two pronouns in a row beginning with the l sound.

e.g. Juan escribe una carta a María
e.g. Juan la escribe a María
e.g. Juan le escribe una carta
but Juan se la escribe (this wouldn’t be correct Juan le la escribe)
others exemples

Se lo dijo a él (He told it to him)
No se lo voy a dar a ellos. (I'm not going to give it to them.)

4.The impersonal se

In some sentences, se is used in an impersonal sense with singular verbs to indicate that people in general, or no person in particular, performs the action

Muchas veces se tiene que estudiar para aprende (Often you have to study to learn.)
Se puede encontrar plátanos en el mercado. (You can find bananas in the market.)

5.To intensify the verb

Magdalena comío tres helados ( Magadalena se comió tres helados) to make to emphasize, or to stress

A final note: Se shouldn't be confused with sé (note the accent mark), which is usually the singular first-person present indicative form of saber ("to know"). Thus sé usually means "I know." Sé can also be the singular familiar imperative form of ser; in that case it means "you be."