can I get a ELI10 on this - If I host a mastodon server, would I need to run this software along side it then and then everyone on my server could see these other platforms posts and such?

Accounts on any supported platform can follow the bot on their platform, and their posts get bridged to all the other ones as a subdomain of the bridgy bot. e.g. a mastodon instance with a handle for every bridged account.

It changed to require opting in to bridging because many mastodon users got very mad about it.