uucp is a suite of programs that used to make the Usenet work. It's now largely redundant, but could be useful in some situations where internet is not available.
uucp handled point to point connections over modems. You set up accounts to allow serial line login from other sites. You put the login info in your local uucp config files - phone no, username, password, times available ... for the hosts you were going to connect into. The actual uucp command copies files. When you use the command it queues up the data to be copied or a request for the file. When the next dialup time arrives (can be set to immediate), your system rings theirs, logs in, and passes stuff between. Mail was overlayed on this. News (newsgroups) were overlayed onto mail.
The really big difference to internet (apart from speed) is the addressing. Because it is point to point, if you want to contact a host that you don't have a direct contact agreement with, you have to know the connection map. email addresses were hostA!hostB!hostC!hostD:username. (That ':' may have been something else - can't remember). Your message was uucp'd to hostA, then to hostB ... There were major hub sites like DEC, HP, Phillips, and several universities, mainly in the US, that paid for the long haul phone calls. Others just had to pay local calls (free in the US).
It was surprisingly quick. I was in the UK when one of the first space shuttles went up. Someone at NASA who could see the launch pad was typing reports into news. We knew the launch had happened before the BBC put out a news flash. Those in Australia were less fortunate. They had to wait for the weekly tape to get there by ship.
As for sendmail config, you only need to add uucp entries if you're using uucp connections. There is stuff in there to allow uucp over IP, but that was for the early days of the transition from uucp to IP in the early '80s.