These fittings are Sch 40 PVC Couplers with FPT (female pipe threads) on each of the two sides. Insert male threaded Schedule 40 pipe for a secure, coupled, fitting. A PVC Union allows you to adjust plumbing and piping after installation, the union is useful if you ever need to dis-assemble your plumbing for cleaning, maintenance, or to fix a drip.

A union is handy as it is easy to seal and take apart compared to a coupler - it's easier for dis-assembly, making regular maintenance + pump cleaning a breeze.