Sixteenth notes split every beat into four, counted “1 e & a 2 e & a”. They are what funk strumming, fast picking and most busy rhythm parts are made of, and keeping them even is a core guitar skill.
The metronome above plays four subdivisions per beat at 70 BPM, with the beat louder than the subdivisions so you never lose the “1”. Strum down on the numbers and “&”, up on “e” and “a”, keeping your hand moving even when you skip a strum.