It's to do with the beats in the bar/ time signature. The three types i think you mean are simple, compound and free. In simple time the conductor would wave the baton for every beat e.g. in 4/4, 2/4 or 3/4. In compound time, each beat is 3 quavers, e.g. 6/8 time, 12/8 time etc. Free metre is found in 20th century western art music and the beats just pretty much fall however the composer wants them to. More info about this will be in any grade 5 theory book.