"Momentum" is a force that makes things that are moving continue to move. You can use this word to describe physical forces. But it's more commonly used for describing success. If you start to be successful at something, like winning games or getting high sales, it's easier to continue to be successful. This is called "momentum":
The tablet computer market is really starting to gain momentum, now that competitiors to the iPad are set to be launched.
To "keep up" your momentum means to continue to succeed.
Other important words to learn with "momentum" are:
- gain momentum
- regain the momentum
- lose momentum
- have momentum