What Plato tries to explain, how people’s opinion cannot be changed unless they experience it at hand.
People who have experienced God cannot express or explain it because the listener would not have experienced similar experiences.
Before going to this theory.

Let us know why we cannot see God?
You cannot see God because he lives in another dimension.
Say, you have the power to enter a 2 dimensional world which is flat and you are able to interact with the chosen one and try to bring that person to a 3 dimensional world and make him experience the Earth (three dimensional) and leave him back. The experienced person if he tries to explain the three dimension experience to a two dimensional crowd. Everyone will laugh at him about his experience in the 3 dimension world. They may not be able to visualize or connect the info.
Moreover, the entry and exit of you will not be noticed by the people in a two dimension world.
Now you know the presence of God will not be experienced by us because he may be living in a multi-dimensional world.
The above understanding will help you to understand the “Allegory of the Cave”

Allegory of cave is a bunch of slaves are chained on the wall from birth and above the wall there is a passage with an opening. where people travel under the light of fire. The fire allows the shadow of the traveler from the top of the wall to fall in front of the opposite wall. The slaves who are tied can only experience the shadow on the opposite wall thinking that is as true being.
You unchain one of the slaves and take him outside to experience the real world. Now he knows the world is open and the light can create shadows and those shadows only they had experienced so far. Now If he goes back to explain his experience about the world he saw, the other slaves will not understand what he was telling. It is more like expressing 3 dimensional experiences to a person living in a 2 dimensional world.

Hence experience alone can enhance true knowledge.