i dont understand what is does
It prevents its stats from being lowered.
Example: If an opponent used growl, the Pokemon with clear body will not have its attack stat lowered, leaving it with the same amount of attack power as it had before.
Based on experience and this source.
The Pokemon's stats can't be lowered by the target's ability or moves. No stat drop in Attack, Defense, Special Defense, Special Attack or Speed. However moves such as Superpower and Wood Hammer cause self-inflicting stat drops. These will happen.
It prevents your stats from being lowered by another Pokemons moves or abilities.
For an example, Intimidate will not lower your attack if you have clear body.
This does not affect moves you use. Meaning if you use a move like Hammer arm and you have Clear Body, you will still lose speed.