Beat is that steady repetition of sound; your heart, the ticking of a clock, the chug-chug of the washing machine. Add sounds to it (longer and shorter), and we get some rhythm; a clothes dryer with shoes in it, marbles falling down stairs, the creaking of heating pipes. Finally, meter is how those rhythms repeat and can be organized. This helps give structure to music.
Meter changes in music can be a powerful tool! Notice the repetition of some musical or rhythmic ideas and then especially notice where the "feel" of the beat changes or the style of the music.