The WPA was the Works Progress Administration or, later, the Work Projects Administration. (It was the same program, but the name changed slightly.) It was a massive public works program started in 1935 that was intended to help people who had lost their jobs due to the Great Depression. WPA projects ranged from big public projects (there's a bridge in a park near my house that was build by the WPA) to more surprising artistic projects. For example, check out the Federal Art Project:
Because it was meant to help people out, it wasn't as focused as a business organized for profit. It was hard to get fired from the WPA. That's why it's so surprising when Bob Ewell loses such a job.