What is "PARALLEL BUILD" in docker hub private registry?

While trying to signup with docker hub by selecting a suitable plan, I see pricing is based on Private Repositories Required and Parallel Builds desired.

What is a PARALLEL BUILD in this context?

PS:
After a bit of an internet search, I found that docker hub can pull up my source code from external repositories and build an image by itself and later publish the same into Hub. If this is true and I don’t want to use docker hub build service, can I ignore the PARALLEL BUILD part entirely?

Source: StackOverflow