what is rem in css

/* CSS3 introduces a few new units, including the rem unit, which stands for "root em". If this hasn't put you to sleep yet, then let's look at how rem works. The em unit is relative to the font-size of the parent, which causes the compounding issue. The rem unit is relative to the root—or the html —element */body { font-size: 12px; margin: 0; padding: 0;}
.one, .two {
  border: solid #666;
  border-width: 10px; border-width: 0.01rem;   /* 1vw */
  border-radius: 30px; border-radius: 0.03rem; /* 3vw */
  font-size: 20px; font-size: 0.02rem;         /* 2vw */
  padding: 20px; padding: 0.02rem;             /* 2vw */
}