A build is an attempt to compile all of the source code of a program into an executable file or binary. In other words, to build source codes means to put individual coded components of a program together. The build is a process that outputs a final program.
Try to imagine you ordered a furniture that is packed in a box (solution/project) and consists of many parts (source codes). After you unpack a box, you have to assemble (build) all parts (source codes) together before you can use the final product (program).
The output of a build is a program that has specific version assigned, for example 22.214.171.1248061001. If you do another build, the new output is a program with higher version, for example 126.96.36.1998061002.
Build as a version (process output)
Very often people use the term "build" to refer the specific version of a program:
X: "Hey, what build should i deploy to the production server?"
Y: "Deploy 188.8.131.528061002. It has been tested and approved as ready for migration."
In the situation above, one person is asking which specific version of a program (compiled source codes) should be installed to the server. The second person is answering that the program with version 184.108.40.2068061002 is to be installed.