A gene is actually a piece of DNA that makes up the chromosomes. The way a gene works is that the cell produces a copy of the gene in a molecule of messenger RNA, and the messenger RNA is then used as a blueprint for creating a protein. The protein could be an enzyme, a hormone or any of a  number of other cellular factors.You can find a more detailed explanation of this process here .

In many cases the location of a particular gene within the cell's DNA is not known, but in some cases you can find that the cell produces a particular messenger RNA strand in a particular situation or set of circumstances. So if you isolate this particular messenger RNA you have a kind of "negative" of the DNA that produced it (sort of like the negative that was made in photography, before everything became digital ).

Reverse transcriptase is an enzyme that knows how to take a sequence of RNA and copy it back into a piece of DNA. So you take the RNA that was produced and convert it into a piece of DNA, and you have just cloned a gene. This DNA can then be used for further research into the gene function.