Trailing slash after domain has no significance whatsoever, but a trailing slash after a page does matter, though most webservers redirect one to the other.
I like to omit trailing slashes if the webserver fixes them to make the URLs take up less space or fewer bytes.
When linking between pages, if your page ends in a slash, relative links go to ./, but otherwise they go to ../. That is, on /about linking to mission goes to /mission, not /about/mission.