The three stages of production are actually quite easy to get your head around once you know what they are. Here's a pretty cool diagram showing them:
So, let's have a look at the three stages of production in more detail:
This stage is about gathering raw materials for whatever it is that needs to be made. In the above diagram they use the production of a computer, so let's go with that. So the primary stage would involve extracting materials like chemicals for the plastic for the outer casing.
The secondary stage involves the transformation, or manufacture and assembly of those raw materials. So that would mean making the chemicals into the plastic for the casing and assembling them around the other parts of the computer.
The final stage of production is the transporting of the goods to be sold, so in this case the delivery trucks taking the computers to a warehouse and the company who owns them advertising them, eventually leading to them selling.
Hope that's helped you with the three stages of production! I found this video on YouTube which gives you some examples of the business sectors that are primary, secondary or tertiary which might help (the music's a little odd though!):