Like all strands of nucleic acid, mRNA is directional. One end is the 5' (five prime) end: the other is the 3' (three prime) end.
When an enzyme uses messenger RNA's sequence to build a protein (the process is called translation), it starts reading from the 5' end, until it encounters a "start" codon. Amino acids are then added to the protein until the enzyme encounters a "stop" codon (three nucleic acids which tell the enzyme to stop). Anything beyond this stop codon is part of the 3 prime untranslated region (3' UTR).
In a diagram:
start stop codon codonMany functional elements occur in the 3' UTR:---------|-------------------|--------- 5'-UTR translated RNA 3'-UTR