Custom weighting in a WCPI pool lets you control how exposure is distributed between the two assets in the pool. For example, you can give a more volatile asset a smaller weight (say, 20% instead of 50%), so that swaps don't move its price as dramatically inside the pool.
Important nuance: custom weighting doesn't automatically reduce risk or impermanent loss. Your actual outcome still depends on the market behavior of both assets — weighting changes how capital is distributed and how the pool reacts to price movement, but it doesn't guarantee lower IL or lower overall risk.
Pool risk for LPs in WCPI is a more involved topic than this short answer can cover, and it's worth treating it as one tool in a broader strategy rather than a switch that "reduces risk".