Memo is short for memorandum, meaning "a short note serving as a reminder."
Memo to self: wouldn't it be great if I could send a short note serving as a reminder to myself really easily, whenever and from wherever ... well, it turns out that I can. Sign up with jott.com, and then call a special number, dictate a message and find it transcribed into an email in your inbox within a couple of hours. You can send memos to yourself or to others using their system. They recognise your phone numbers (using caller-id) so you don't have to authenticate yourself when you call.
Memo to self: spend less money ... this is a useful short note about which we should all be reminded on a regular basis. However, for startups who have outside investment (and therefore are spending other people's money), this is a particularly important memo. Check out Seth Levine's brief and interesting post on the topic.