This is the 8137th most frequent Polish word.
oczyści
"Oczyści" means "will cleanse" or "will purify" (third person singular, future tense).
The verb 'oczyści' here is in the future tense indicating a planned or expected action.
Nowa firma oczyści stare zanieczyszczenia z tej rzeki.
The new company will clean up old pollution from this river.
The verb 'oczyścił' indicates the past-tense action of cleaning or tidying up performed by 'he'.
On oczyścił swoje biurko po pracy.
He cleaned his desk after work.
The verb 'oczyści' here is in the imperative form, used as a direct command or encouragement.