The prefix di- makes it passive. "Mereka menangkap polisi" = "They catch the police", "Mereka ditangkap [oleh] polisi" = "They are caught by the police." The "oleh" ("by") is optional, in Indonesian, you can just say "They are caught the police" and it'd make perfect sense, so it can be a bit confusing for an English speaker at first. ^,^
That terrible English articles... 50% of my mistakes are not made in the language which are being learned now. Like here: "By police" -- no, by THE police. Wrong answer! I know, that it is a real mistake, but... I'm learning Indonesian, not English :-( Maybe, these moments should be forgiven :-)
But you are choosing to learn it in an Indonesian through English program. The same thing happens to me when I do Catalan and Guarani. Spanish is far from my best language, unfortunately. Sometimes I will warm up by doing a bit of the Duolingo Spanish just to remind myself.