Nexus was first created as a tool to bridge the gap between CI and QE test automation. However, as it evolved, it took the shape of a tool that could be used by others with their custom config and perform tasks using multiple other tools with a single command line.
In short, Nexus is designed to connect multiple tools and test frameworks for any pre-requisities, tasks or post requirements with Jenkins. Nexus can also be executed using its commandline interface.
Full Docmentation is here.