The wheels ride on the rails. The bolts that hold the wheels in place are loose, or have loosened. At least one of those bolts has an eccentric fitting which can be rotated to tighten the fit of the assembly to the rails. Snug up all the bolts, then turn the eccentric with a wrench until there is no looseness, but not so tight as to cause binding.

Once you are certain you have an easily moved carriage, tighten the eccentric bolt to keep it in place. It's possible you have two of these eccentrics. Look for a hex shape between the wheel and the rail.