Dynamical friction (or Chandrasekhar friction) is drag on an object passing between other objects because of the gravitational attraction. An object moving relative to a bunch of others draws them toward its path, decreasing the distance and thus increasing the gravitational attraction between the object and the others, and the object is slowed, momentum conserved by the others being dragged along the same direction. The bunch, on balance, end up with a bit more velocity toward the path of the object's travel. (Running the scenario in reverse, they'd start out with a bit of velocity away from the object's path, increasing their distance to it as the object is nearing them, reducing their gravitational attraction to the object after it passes.)
Dynamical friction is theorized to affect galaxy collisions, due to its effect on the constituent stars, making a galaxy merger more likely. Dark matter dynamical friction (slowing in this manner by dark matter, occasionally abbreviated DMDF) is thought to affect compact object mergers, and also satellite galaxies and globular clusters, making them more likely to merge with their host galaxy. If the fuzzy dark matter model is valid, its characteristics could significantly affect the timescale of such infalls.