Zaparkujcie samochód na głównym parkingu, zanim plac się zapełni.

zaparkujcie is the 2nd person plural imperative of the perfective verb zaparkować (“to park [successfully]”).

• 2nd person plural (“you all”) because you’re addressing more than one person.
• Imperative mood to give a command.
• Perfective aspect (zaparkować) signals that the action should be completed (you actually get the car parked).

samochód is the direct object of zaparkujcie, so it’s in the accusative singular.

• samochód is an inanimate masculine noun, and for inanimate masculines the nominative and accusative forms coincide, hence samochód.

Here na expresses location (“on/at the main parking lot”) and governs the locative case.

• parking → locative singular parkingu
• główny → locative singular masculine głównym
  So na głównym parkingu literally means “at the main parking lot.”

zanim is a subordinating conjunction meaning “before,” used to introduce a full clause with its own verb. Example: zanim plac się zapełni (“before the lot fills up”).
By contrast, przed is a preposition, so it must be followed by a noun or an infinitive, not a finite verb. You could say:

• przed zaparkowaniem (before parking)
• przed wypełnieniem placu (before the lot fills up)
  But with zanim you directly link two clauses.

się is the reflexive pronoun needed for the verb zapełnić się, which means “to become full” or “to fill up (oneself).”

• zapełnić alone is transitive (“to fill something”).
• zapełnić się is intransitive/reflexive (“to become/ get filled”).

Polish word order is relatively flexible. You could also say:

• zanim się plac zapełni
• zanim plac zapełni się
  All three versions mean the same. The chosen order simply places się right after the subject plac, which is very common stylistically.