15 seconds is a bit steep, even for an elevator pitch, but if that's all you have, then I'd recommend describing the problem you are solving, and the total addressable market size.
The problem should be described as a short story made of simple words, rather than a technical description with fancy words. As a random example, let's take... oh I don't know... Dropbox. "We are building a software framework that can perform file backup and synchronization across devices and platforms faster and easier than ever before..." - that's it, you just lost your audience. "Have you ever needed to copy a bunch of files from your computer to your phone? A nuisance, isn't it? And what if you need these files urgently when you're away from your computer? Bummer. Now imagine that it's all done automatically - you just take out your phone, and all your files are already there, as if by magic. That's what we do (and that's just for starters)".
The TAM size should be well over $1B if you want to get attention from an investor, otherwise it;s not worth mentioning (and probably not worth your time looking for investment in the first place).
If you still have a few seconds left (or if you see that the other person is interested), you can add your strongest card to the mix. That could be a good traction, revenue growth, an exceptional team, an endorsement from someone highly credible, strategic partnership, etc.