SOLIDWORKS API (Application Programming Interface) is a programming interface to SOLIDWORKS software. Programmer can directly access to SOLIDWORKS functionality such as create sketch, extrude, create new part, create drawing, modify dimension and thousands more other function to create a macro or customized program. This allowed SOLIDWORKS user to harness the full capability of this powerful software. A lot of partner product utilize this API to interact between their product and SOLIDWORKS. For instance, MasterCAM created an add-in which is MasterCAM for SOLIDWORKS which allowed MasterCAM user to streamline their design and manufacturing process. This sort of add-in is one of the many possibilities what using SOLIDWORKS API can achieved. However, it is not easy, not easy at all. In order to create such fancy add-in, one must possess extensive knowledge in all aspect of mechanical theory, mathematical calculation and programming skill.
Back to when you are still studying, you might be wondering when you are going to use all the algebra and geometries learnt in mathematics. Well, those who are creating SOLIDWORKS and SOLIDWORKS add-in possess a strong knowledge in these topics. You must know how to deal with integration, derivative and matrices when creating engineering software. Creation of software is tough, same as designing mechanical products. You must have experience, more experience and much more experience combined with many years of practices to create or design your own product. However, nothing is impossible when you have a strong will.
The beginning is always the toughest, SOLIDWORKS API is consider as one of the starting points for SOLIDWORKS user to learn programming. This is because the interface is what you are familiar with, and the functionality and behaviour are more or less predictable. As a conclusion, SOLIDWORKS API is a programming interface to SOLIDWORKS where you can create macro, add-in or standalone program. The functionality of these programs varied from design automation, rendering and simulation. There is no limit what these programs can do, the only limit is your own imagination.
As an example, below is the IME InterX which is an add-in for SOLIDWORKS created by IME: