Would somebody please advise me how to correctly interpret how the scale factors of maps are read?

I have a question which say somebody walked 8 km, then used a map with a scale of 1:25000, then it says on the first part of the map the distance was 18.4cm, find the corresponding distance in km?

So how do I know what the scale represents?

Is it 1cm = 25000 km or 1 km = 25000 km or 1cm = 25000 cm?

Please advise how one is to know?