I came across the term systemic velocities of galaxies. Can you please explain what it means?
It means the average velocity of the galaxy, or the velocity of its center of mass. This can either be an actual velocity inside a gravitationally bound group or cluster, or its cosmological redshift expressed as a velocity.
This is important because different phases of the galaxy move differently. The galaxy rotates, so stars on one side will have velocities different than those on the other side. There is often outflowing and infalling gas in the galaxy which will move with a different velocity than the stars, etc.
Given that these velocities are often mainly interesting in relation to the general frame of the galaxy, they are mostly given in reference to the Systemic velocity. Of course, we cannot know this precisely, as typically don't know the exact mass distribution of the galaxy in question. When possible, the average velocity of the stars is usually adopted as systemic velocity; other times, it is typically strong emission lines from hot nebulae that are known to occur in association with hot stars are adopted. Other methods can be adopted, too, based on what is practical with the observations and from what is already known about the object.