Hi,
SPB means Swift page builder section. The idea is to reuse blocks of content.
Imagine that all your pages start with a Slider and 2 text blocks below.
Instead of creating this structure in every necessary page you only have to create it once by creating an SPB Section.
Then in your pages you just have to add that specific SPB Section available in the Swift Page builder assets. If you need to change something you only need to do it once.
Let us know if need further explanation.
-Rui