Concept of Gravity
Well, there are simple acts that demonstrate the gravitational force.
Ask you toddler to hold a toy, take it to shoulder level and release it, do not throw it, and it falls down. You may have to repeat couple of times for the child to get in sync with the activity. Be sure, to enact and talk in appropriate voice to hold the child's attention.
You could ask the child to repeat doing it with various toys. Be careful in how you teach your child. Make sure he doesn't start dropping all easy-to-break things around. You can always teach appropriate ways of playing. Do not give up and yell if your child doesn't heed your word.