Great question!
Physical changes change the appearance of something or someone.
For example, let's say 2 cats get into a fight. One is left clean in the end of this, with no scratches or bites anywhere, so nothing has happened to it as a physical change.
However, the other cat is left scarred and with bite marks everywhere. All this has left the cat's once-beautiful coat now ragged and torn up. This is an example of a physical change.
↑ ↑ ↑ Hope this helps! :D