Messenger RNA (mRNA) is an important form of RNA, both in the body and in the lab. Physiologically, it is used as the coding template for proteins, i.e. it directly transcribes the sequence of nucleotides from the DNA template (in a process known as transcription), forming a second complementary strand which is later processed by rRNA and tRNA to form a new DNA strand.
Pre mRNA is the exact copy of the DNA sequence, containing introns and exons. This is not helpful in cloning experiments as bacteria and other prokaryotes do not have the capibility to process this form of mRNA and remove the introns.
Mature mRNA is the product when pre mRNA undergoes processing. This involves removing the non-coding introns, joining the coding exons together, and adding a poly A tail to the 3' end, to form a new strand with an uninterrupted sequence which is then translated into amino acids and proteins. Mature mRNA is required for protein cloning in the lab as it does not require any further processing by the prokaryotes.