The courant number shows you how rapidly the result field is changing in a single timestep in comparison to your mesh size. For example, a courant number of 0.5 would mean that fluid velocity is high enough that a particle would move (I say would move because the software models a continuum, not individual particles) half of a mesh element in a single time step and a counrant number of 1 means moving an entire mesh element in 1 time step. If you are interested, tell me and I will explain why it is bad but for now just know that your courant number should be below one.
To lower your courant number you should reduce your time step so that the time interval is smaller and the ‘particle’ therefore travels a smaller distance during the timestep. You could also reduce mesh density but this is usually a bad idea unless you have chosen an unnecessarily dense mesh to begin with.