The 3' and 5' untranslated regions are the parts of the mRNA directly before and after the part that encodes the actual protein. So they are themselves not translated into amino acids.

What they actually do can vary, but essentially they can provide places for other things to bind and influence what happens with the mRNA. There are some fancier cases like riboswitches, but you don't see those in humans. The stuff at the start and end of the mRNA also determines stability of the mRNA.