Yes it's possible to burn food in a slow cooker if you don't keep enough water in the slow cooker to keep the food moist.
If the water evaporates from the slow cooker then the food in the slow cooker could become burned.
Also if you leave the food in the slow cooker for too long then it can also burn the food but it would take between 3 to 4 days of the food being in the slow cooker cooking before it actually burnt.
You won't burn the food like you would on the stove though so using a slow cooker is a good way to cook food while you're busy doing other things or need to run to the store, go to work etc.
You can also add food to the slow cooker and then go to bed and in the morning the food will usually be cooked and ready to eat.
I've never burnt any food using a slow cooker but it's possible if you're not careful.
As long as you keep enough water in the slow cooker then you're unlikely to burn any food in the slow cooker.