Baculoviruses are a a class of viruses which infect only a pretty small number of insects (or their cells in cell culture). To do so, the virus has to infect a cell, take it over to produce the own proteins and DNA to make new virus particles and finally set these new particles free. This is basically what is shown in the image below (from here):
From the proteins encoded by the viral genome, not all are essential, meaning removing them will not do harm to the virus or its function. And this is used in Biotech to produce proteins of choice. One of the non-essential proteins is replaced by the sequence of the protein you want to have produced by the cells. You then take insect cells in cell culture (these are single insect cells which live in a big vessel full of culture medium), infect these cells with the modified virus and eventually harvest the protein from the culture.
The advantage of Baculoviruses is that they only infect the insect cells on which they are specialized so they are a safe way of producing proteins for pharmaceutic applications. If you want to read some more about this, start here.