This is the 635th most frequent Polish word.
odbył
"Odbył" means "he/she/it completed, underwent, or took place."
In this sentence, 'odbył' is used to indicate that Jan completed or went through his military service.
Jan odbył służbę wojskową w zeszłym roku.
Jan completed his military service last year.
Here, 'odbył się' is reflexive and describes an event occurring or taking place.
Koncert odbył się w miejskiej hali koncertowej.
The concert took place in the city concert hall.
In this example, 'odbył' denotes completion of an experience or a formal process like an internship.