mRNA, or messenger RNA, is the bridge between DNA and proteins, where DNA "stores" the hereditary information, and proteins are often the products made from DNA for a variety of purposes including cell function and structure. In the nuclei of eukaryotic cells, DNA is transcribed into mRNA, essentially a copy of the original DNA information. (The transcribed mRNA isn't actually an exact copy, but the reverse complement of the original DNA, kind of like a mirror image.) The mRNA then leaves the nucleus and is translated into protein through ribosomes and tRNA, which are specific carriers for protein building blocks that match up to the mRNA sequence.
There are several advantages to having mRNA as the "bridge" between DNA and protein. First, this allows DNA to be kept in the nucleus, away from degrading enzymes. Second, the existence of mRNA allows for additional regulation of gene expression. For example, multiple copies of mRNA can be made from a single DNA template, allowing for high expressions of certain genes. Also, gene expression can be stopped by degrading the mRNA since more mRNA can be made when gene expression needs to be turned on again.
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