This is the 9919th most frequent Catalan word.
multen
"Multen" means "they fine" (third-person plural present tense of "multar").
In this sentence, 'multen' is a third-person plural conjugation of the verb 'multar', indicating that the subject (police officers) is performing the act of fining other entities.
Els policies sempre multen als vehicles aparcats en llocs prohibits.
The police officers always fine the vehicles parked in prohibited spots.
Here, 'multen' indicates a general action where the implied actors (authorities) apply fines to those breaking rules.
Si els conductors infringeixen les normes de trànsit, els multen.
If the drivers violate the traffic rules, they get fined.
In this case, 'multen' is used somewhat impersonally, suggesting that entities (likely authorities) fine individuals under specific circumstances.