The lat pulldown targets the same muscles and replicates the same vertical pulling down motion as the pull up.
Both can be good exercises to help build back muscle and upper body strength. The lap pulldown is performed using a weight-adjusted cable exercise machine whereas a pull up is typically performed hanging from a bar using your own bodyweight.
The lat pulldown is good if you do not have the strength to pull your own bodyweight or if you want to perform drop sets, as you can adjust the resistance on the machine. As it requires less core and scapula control, it can be easier and more manageable for beginners.