From the previous subject we saw that an application is made on the basis of source code, which again consists of header- and source code files such as C++ files. Since a toolkit very often doesn't contain the source code needed to build the toolkit itself (got it ? The toolkit itself is also build with its own source code), it only ships the header files.
The header files with it's appurtenant library files are enough for you to build an application with the features of the toolkit. You don't need the toolkit's source code to get along.
Very often you will find the header files in the include sub directory, such as;
E:\Toolkits\LibraryX\Include
The header files are very important to the compiler, and you will specify where the header files are located during setup of your compiler.