Why don't you show us your platform and let us tell you what we think it does?
"It's a platform" is never a good answer to "what does your app do". The question is not necessarily "what does your app do" but "why should I care about what your app does".
Note that this new question becomes relative to who is asking the question. In this case it is an investor who's goal is to make more money than he put in. If you know this person also likes novel ideas, is very tech savvy, or just likes straight numbers - all of these things should affect your answer.
Mysupersite is a marketplace for
freelancers to buy and sell web-based
services such as graphic design work,
and copy-writing. It's similar to
(well known site) but we offer a
credit system where users can earn
credits to buy services by offering
services of their own. We make money through transaction fees.
"My website is a platform. You can
seek for people to do something for
you, and you need to pay them; on the
other hand, you can do something for
others by spending your time,and you
"platform", "people", "something", "them" are all vague and they mean nothing. Essentially you are using a whole lot of nothing to tell me nothing.
It actually is probably just because you are hesitant to divulge the full extent of what you are working on - another mistake.