Yes the slope has units!
Slope = rise/run
Since you are plotting force against mass, the rise in the graph is the change in force while the run is the change in mass.
Therefore, slope= Force/Mass
I am assuming that you are dealing with the SI system, which means force is measured in Newtons (N) and mass in kilograms (kg).
This means that the units of the slope are N/kg. However a Newton is also equal to one kg*m/s^{2}, so if we divide this by one kg we are left with a slope with units m/s^{2}, which acceleration is measured in!
This makes perfect sense considering that F=m*a. If we divide force by a certain mass we have F/m=(m*a)/m. In other words F/m=a. And since the slope of this graph was calculated by dividing F/m we know that it represents acceleration. That's proof that the unit for the slope is m/s^{2}.
