Se is undoubtedly the most versatile of the Spanish pronouns. As you learn Spanish, you will come across se used in a variety of ways.
For the beginner, it isn't necessary to learn all those ways, although it can be helpful to be introduced to its various uses to help avoid confusion when you see it used in a way you haven't studied yet.
As the equivalent of the English passive voice. By using se, particularly when discussing inanimate objects, it is possible to indicate some sort of action without indicating who performed the action. Grammatically, such sentences are structured in the same way that sentences using reflexive verbs are. Thus in a literal sense, a sentence such as se venden coches means "cars sell themselves." In actuality, however, such a sentence would be the English equivalent of "cars are sold" or, more loosely translated, "cars for sale."
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.
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. When se is used in this way, the sentence follows the same pattern as those in which the main verb is used reflexively, except that there is no subject to the sentence that is explicitly stated. As the examples below show, there are variety of ways such sentences can be translated to English.
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."