cDNA is short for complementary DNA. It is DNA which has been reverse transcribed from mature mRNA. DNA is much more stable than RNA, so cDNA is used to make a copy of the DNA (cDNA also stands for copy DNA go figure
Okay, eukaryote DNA (we're eukaryotes, by the way) has segments in it that don't really code for anything useful. These segments are called introns. The segments between introns that DO contain useful code are called exons. Normally, when DNA is transcribed to make mRNA, the mRNA then goes through processing to splice out the introns and glue the exons together. The result is a piece of mRNA that is ready for translation in the ribosomes.
Prokaryotes (bacteria) don't have introns, and they don't have any mechanisms for removing introns from eukaryotic DNA. Hence, if you want to insert a human gene into a bacterium, you have to remove the introns yourself. But how do you do that You take a segment of mRNA that has already been processed, and reverse transcribe it into cDNA. The cDNA lacks the introns that were part of the original DNA and is ready for insertion into a plasmid.
hope this helps you!