[gemy]

LP stands for Luxating Patella. It comes in four grades - 1-4 the lower numbers being less severe.
And 5mths old and jumping off the couch, unless your couch is 6 inches off the floor; I'd stop this activity right away.
A mature healthy bone structure should handle normal jumping (unless you have a very tiny dog), but at 5mths old the growth plates do not close until at least 12months. So as wee pups we need to safeguard their boney structure. That means good quality exercise, good food, and limited jumping and especially jumping onto slippery surfaces like hardwood floors.

The kneecap (patella) will slip out of alignment, and in the earlier grades will self adjust back in; but there is a temporary "locking" of the knee and the inability to flex or extend.

Take your pup into the vet and explain symptoms. As this is a very common problem in the toy breeds, your vet should be very familiar with it. There are specific exercises you can do to help strengthen the upper and lower thigh.