Most watercolor techniques require the paper to be soaked in water and therefore the ability of the paper to not wrinkle and curl is of prime importance.
The heavier the paper the better it will resist wrinkling and curling but other factors affect the papers performance.
Paper can be made by different methods, the forming in a rectangular mould is best. Papers made in moving rollers are cheaper but have a grain which warps. Also cotton rag content is important – it adds the strength of long fibers.