This is the 1143rd most frequent Polish word.
skończył
"Finished" or "ended."
Here, 'skończył' represents the past tense of the verb 'skończyć' (to finish), indicating completion of the action of finishing work by Mateusz.
Mateusz skończył pracę wcześniej.
Mateusz finished work early.
In this sentence, 'skończył' is part of a reflexive verb 'skończyć się' (to end), which describes the movie coming to its conclusion.
Film właśnie skończył się.
The movie has just ended.
Here, 'skończył' is used to denote the completion of the activity of finishing a project in the past.