Diagnosis
The locking collar that secures the handle to the mop body has worked loose. It's a plastic ring you twist to tighten.
How to Fix It
This is an easy retightening job. 1) Look where the handle meets the main mop body. 2) You'll see a textured plastic ring. 3) Grasp the mop body firmly with one hand and twist the ring clockwise with the other until it's snug. Don't force it—hand-tight is enough.
If That Didn't Work
If the wobble is actually in the hinge where the handle folds, check the two silver hinge pins. They can sometimes work partway out. Gently tap them back in flush with a hammer.
Heads Up
When unfolding the mop for use, make sure the handle is clicked fully into the upright position before you tighten the collar. A partial latch will feel loose.