I try to send videos to friends but I get a message in red saying that one or more receipents cannot receive.... what's this all about?

The message itself originates from a mail server and is often pretty cryptic for the bigger part of it, but somewhere in between that message is a reason why your message has been rejected. It might say your recipients mailbox was full, for example - keep in mind that a video is pretty big, and e-mail isn't really designed to transfer huge files over the internet.
Another common occurance is a typo in one of your recipient's adresses. You might then get a message that the hostname could not be found (the part after the at-sign), that the user is unknown to the mail server, etc.
It's important to clearly read what's in that message you're getting. Somewhere in the garbled up technical jargon is actually some decent English that explains to you why you're getting this message.