Relative length units specify a length relative to another length property. Relative length units scale better between different rendering mediums.
Source: W3School
rem is a CSS unit that is relative to the font-size of the root element.
0.25rem is 4px
0.50rem is 8px
0.75rem is 12px
1rem is 16px
Here’s a helpful chrome extension to help you convert px to rem called PxtoRem: https://chrome.google.com/webstore/detail/pxtorem/ldbemkmafmibhpncgagpegmhbhpolnlp?hl=en