Phrase: participate in the contest, enter the competition.
Let’s break down the phrase from the side of the beer can.
Osallistu is the second-person singular imperative present of osallistua, a verb which means ”to participate in,” ”take part in,” ”partake in,” or ”be involved/included in.” Osallistua is an intransitive verb which usually takes the illative. (See osallistua conjugated here.)
Kilpailuun is the illative of kilpailu which means ”contest,” ”competition” or ”race.” In the illative it has the meaning of ”into the contest.”
Osallistuja is a noun created from osallistu that means ”participant.”