Some of the basics of galaxies rotation can be understood using basic mechanics (and a lot of assumptions that are probably not completely right but give a good aproximation).
First, lets assume that the galaxy is a rotating, homogeneous plane disk (plane means that the radius $R$ is much bigger than the height $h$, which is mostly true for spiral galaxies, for example, Milky way has $R=100\,kpc$ and $h=0.6\,kpc$).
Let's focus now in a single star located at a distance $r$ from the center of the disk, and let's assume it's moving in an uniform circular motion. The velocity is then perpendicular to the position vector from the center to the star, and its modulus is
$v = \sqrt{a\times r}$
where $a$ is the centripetal acceleration. This acceleration is caused by gravity, and it can be shown that it's caused only the mass of the disk inside $r$: think of this as a <PERSON> law for magnetism, but applied to gravitation. As in magnetism, this "Gauss law for gravitation" allows us to change the mass of the disk inside $r$ for a point of the same mass located at the center. 24
With this, the acceleration is
$a = \frac{GM}{r^2} \Rightarrow M = \frac{v^2r}{G}$
Where $M$ is the mass of the disk inside the distance $r$.
With this, you can infer the total mass in the disk measuring the velocity of stars at different distances from the center, and make a plot of distance $r$ vs. the mass of the disk $M$ inside that radius. This is called dynamical mass, since it was obtained from the dynamics of the system, and represents all the mass that contributes to gravity.
So far so good. The problem is that you can also calculate the mass of the galaxy using light: making some additional assumptions, you can relate the ammount of light with the number of stars, and that number to the mass. If you do this using only the stars inside a certain radius, you end with the same plot as before, but now with the mass obtained from the ammount of light, hence called visible mass.
As you may know, visible and dynamical masses do not match, and that's why the introduction of a new type of matter is needed, a matter that affects gravity but can't be seen, and has been called dark matter.
Going back to your question, dynamical mass represents the total mass affecting gravity, and from rotation curves you can obtain the mass enclosed inside a certain radius. 24