Bars, and the two numbers

The beat is steady, but the beats are not all equal.

Listen: the first of every four is marked here, an octave up, so the grouping is impossible to miss. In real music nobody has to mark it — you hear the grouping anyway, and once you have noticed it you cannot un-notice it.

Count along, one two three four, one two three four. Four beats to a group. That group is a bar.