Cheese starter cultures are a form of bacteria used in the production of cheese. They are added to the milk at the start of the cheese making process (not for all cheeses though, there are exceptions such as Haloumi) in order to determine taste, smell, texture and mouth feel.
The main cheese starter cultures are Thermophilic and Mesophilic. Thermophilic cultures can handle being heated to higher temperatures, Mesophilic cultures suffers at higher temperatures.
There are also specific culture strains made by commercial manufacturers which fall into each of these types (Therm and Meso) and each subtly changes the results you will get with your cheese. Your recipe should tell you which culture type you need.