Here's a breakdown:
* Por: This preposition translates to "by," "for," or "through."
* el: This is the definite article, meaning "the."
* que: This is a relative pronoun, meaning "that" or "which."
* se establecen: This is the third person plural present indicative of the verb "establecer," which means "to establish."
Therefore, the phrase describes the reason or purpose for which something is established.
Example:
"Las reglas por las que se establecen los equipos deportivos son estrictas."
Translation:
"The rules by which sports teams are established are strict."
In this example, the phrase "por las que se establecen" refers to the rules that govern the establishment of sports teams.