At AS and above, we start writing units in this way. If you think of velocity as distance over time, you can see where the unit m/s has come from: distance in m divided by time in s to give m/s.
If you take maths, you'll know that 1/s is the same as s^-1 (to the power of -1). Multiplying 1/s by m gives you m/s which is thus equivalent to ms^-1.
When writing in this format, make sure not to include the /. e.g. 5kg/m^3 becomes 5kmg^-3, and 3m/s^2 becomes 3ms^-2.