A build is a prepared version of a project made from its source files. It is like assembling separate pieces into a package that can be tested or delivered.
For a website, the build step can combine code, styles, and images into files a browser can load efficiently. If the build reports an error, the project may have a problem that needs fixing before it can be shared.
Building a project does not automatically put it online. It creates the prepared files; a separate deployment step makes them available to other people.