The music has two, three or four beats in each bar, and each beat is a crotchet (1 beat note) and each beat can be divided into 2. If you try saying the word 'sim-ple' in each beat, and it fits with the notes, then you are listening to simple time.
Simple time signatures are written as 2/4, 3/4 and 4/4. 4/4 can also be written as C, which stands for 'common time'.
Watch the accordionist's foot and how the word 'sim-ple' fits within each beat.
'Sim-ple' still fits this modern song.