If I understand it right it’s basically polishing your external appearance to a high degree. Hair, skincare, etc… but I’ve seen it mostly used to describe people using the technique transactionally. Like incels doing it with the expectation that doing so will result in a girl appearing for them.

I think it’s incel for having a shower.

I think the way to read the whole -maxxing suffix is “to maximise what comes before”

So lookmaxxing is maximising one’s looks. I find the usage of the double x irksome, personally.