Spondylolisthesis is an intimidating word at first glance, but all you need to do is break it into 4 sections: spondy-lo-lis-thesis. Then it's not so hard to say. Spondylolisthesis is also not so hard to understand if you think about its Greek root words: spondylo means "vertebra" and listhesis means "to slip."
Spondylolisthesis is when one vertebra slips forward over the vertebra below it. Most often, that happens in the low back (lumbar spine) because that part of your spine bears a lot of weight and absorbs a lot of directional pressures. In other words, your lumbar spine has to move quite a bit (rotate in various directions) while carrying your body weight. Sometimes, this combination can put so much stress on the vertebrae that one of them slips forward.
Some activities make you more susceptible to spondylolisthesis. Gymnasts, linemen in football, and weight lifters all put significant pressure and weight on their low backs. Think about gymnasts and the positions they put their body in: They practically bend in half backwards—that's an extreme arched back. They also twist through the air quickly when doing flips and then land, absorbing the impact through their legs and low back. Those movements put substantial stress on the spine, and spondylolisthesis can develop as a result of repeated excessive strains and stress.
The x-ray below gives you a good example of spondylolisthesis. Look at the area the arrow is pointing to: You can see that the vertebra above the arrow isn't in line with the vertebra below it. It's slipped forward; it's spondylolisthesis.
Doctors have 5 different grades for spondylolisthesis, depending on how far forward the vertebra has slipped. During your appointments, then, you may hear your doctor talk about "grade I spondylolisthesis." For a full explanation of what the grades mean and how the doctor determines your grade, please read Exams and Tests for Spondylolisthesis.