Meter is the natural division of rhythms into equal sized groups.
For example, much of today's music is written in 4 beats per measure
and is one example of how the term meter
is used to divide music into 4 beat per measure.
So, within that time frame of one measure of 4 beats per measure
you can have 1 whole note or 2 half notes or 4 quarter notes.
You change meter to divide the music into 2 beat per measure.
So, within that time frame of one measure where there is 2 beats per measure
you can have 1 half notes or 2 quarter notes or 4 eighth notes.