It essentially reduces the time needed to get a project off the ground for developers who like to build on top of amazing frameworks. You could go and collect these open source publicly available frameworks and merge them yourself, but downloading UnderStrap saves you countless hours of development.
A team led by Holger built the Bootstrap 3 and then the Bootstrap 4 version. You can find the names of the people involved on the home page of this site.
We recommend starting with a child theme and editing that way, we wrote a guide on how to do that.
If you need help with getting started with UnderStrap then there is a Discord channel set up with developers watching and offering to help.
You can join the channel using this URL: https://discord.gg/xARbF3f
UnderStrap is distributed under the terms of the GNU GPL version 2
This means you can take UnderStrap, edit it and use it how ever you want to. Giving us a shout out in the credits of your project would be greatly appreciated.