The United States Senate has 6-year terms. There are a hundred members of the Senate so every six years 34 of them are up for reelection. This is one of those years. Normally this is just one vote extra for this cycle. But this time around in 2016 out of 34 Senate seats available 10 are currently held by Democrats and 24 are held by Republicans. The Democrats need to send it back plus another 10 seats to make it filibuster-proof.
The state of Colorado is already purple. So when you get a conservative Tea Party candidate up for Statewide vote the Republicans have basically forfeited that seat to the incumbent Democrat.