Parity of esteem means that everyone should be treated equally, no matter what they look like or where they come from. Imagine you and your friend both love playing soccer. Just because your friend is really good at it and you're not, it doesn't mean that you should be treated differently or looked down upon. You both deserve the same amount of respect and attention.
The same thing goes for people who have different kinds of illnesses or disabilities. Just because someone has a mental illness or a physical disability, it doesn't mean they should be treated any differently than someone who doesn't. Everyone should be given the same opportunities and respect, regardless of their differences.