Basically, miniprep is short for minipreparation. It is a procedure which includes you culturing a bacteria culture overnight to get a lot of bacteria that can be centrifuged the next day to obtain a pellet. What miniprep does is to lyse the bacteria so that the plasmids can be isolated. This involves the usage of a few buffers that breaks the membrane of the bacteria, washing the DNA and eluting the plasmids.
The plasmids can be used for restriction digestion to check for inserts that you need or for you to insert DNA inserts into it. Basically, you make use of restriction enzymes, which recognizes restriction sites on the plasmid and cut them up. The same or different restriction enzymes can be used on another DNA which you want to insert into the plasmid. Ligation is then performed to ligate the DNA insert into the plasmid.
I hope this is clear enough for you? You can look up in google for more details. Or else, provide us with more details on what you do so it will be easier for us to tell you what is going on. ALternatively, you can always ask your supervisor.