Elaborate or remind, but try not to repeat yourself.
Let's say you introduce a character early on in your story:
Protagonist opened for a handshake, but Character simply stared at their hand, as if it were too much effort. They looked miserable, with dark messy hair, washed out clothes and scuffed shoes.
Then, you meet the character again. It has been most of the book since you met them.
Character was just as anti-social as last time. Protagonist smiled as warmly as they could. Character's hair was the same dark coloured mess and though their suit was still absent of colour, their shoes were only more scuffed and marked, almost giving them more personality.
The idea is to remind the reader of what the protagonist already knows, not what the reader should remember, even though your goal is actually the latter. Most readers will remember when prompted, or will simply acknowledge that this has already been stated.
As a reader with a memory like a sieve, I really appreciate being reminded of character and location descriptions, especially when they tell me even more about someone. In my example, the elaboration of the second paragraph would not have the same effect without the first paragraph. Use that to your advantage.
What I meant by "do not repeat yourself" is to avoid saying the exact same thing. If our second paragraph read as:
Protagonist smiled as warm as they could. Character was just as anti-social as last time. They looked miserable with dark messy hair, washed out clothes and scuffed shoes.
Then readers who do remember will gain very little from the description and even feel patronised - which I imagine is your concern - so make sure you vary the description, even if you are admitting that you've told the reader this before.