Linking with libraries

[Travis]

So I have limited makefile / g++ experience. I am not working with
existing libraries. I see in the directory for the library tons of
files including the .h, then there is a bld with a .cpp, and then I
see .a and .o files.

What is a .a file?

I am looking to use this library such that when I do a #include
"mylib.h", it knows where to find that library and compile against it.

All help appreciated. Thanks.