Go to wikipedia and look reverse transcriptase. All your questions will be answered. But to answer your question, reverse transcriptase is an enzyme that uses RNA molecules as a template to make a DNA version of said RNA. ie the flow of information from DNA to RNA is reversed.
The central dogma... is a terrible name. It was an idea state in the 1950s that information flows one way from DNA to RNA to protein. The use of the word dogma is an ill choice to use. Because science does learn new things. And given that DNA was only just discovered in the 1950s....ie the same decade... the word dogma is a bad idea. SHould have waited a few more decades first
Anyhow... the discovery of reverse transcriptase allow information to flow back from RNA to DNA. The discovery of siRNA.. mean RNA can control DNA. The discovery of prions... lead to current work now where pre-existing proteins can be templates for the folding of newly synthesised protein. DNA methylation allows epigenetic inheritance. Ie changes of certain protein levels (say during starvation) in your body can leave inheritable marks on your DNA (which will then affect the behavior of those gene in your children).
The central dogma now (if it still is used) has a lot more lines leading up and down the chain.