Thematic means 'to do with a theme' — a theme being the one idea that runs through a lot of different things and ties them together. If a museum shows pots from Japan next to pots from Peru next to pots from Egypt, that is a thematic display: the ordering idea is 'pots', not 'Japan' or 'the year 1600'. So thematic is very often used in contrast with two other ways of organising things: by time (chronological) and by place. A teacher may say 'we teach history thematically' — meaning we do war, then trade, then family life, each across many centuries, instead of marching year by year. In this use it describes how something is ARRANGED.

The other common use is about content, not arrangement: 'thematic' points at what a book, film or piece of music is really about underneath the story. 'Thematic elements', 'the thematic heart of the novel', 'thematically rich' — here it means the deep repeated ideas (loss, freedom, growing up) rather than the events on the surface. Film reviews and literature classes lean on it heavily, and it is a favourite word in academic writing, arts writing, and business presentations ('a thematic approach to investing'). It feels formal — you would not usually say it to a friend at lunch. In everyday speech people just say 'it's all about…' or 'it's organised by topic'.

One more, quite separate: in music, a 'thematic' motif is one of the short tunes the piece keeps coming back to. And in language study, linguists use 'thematic' as a technical term about the roles nouns play in a sentence (who does, who receives) — but your students will almost never meet that one.