Standing water in a crawl space is liquid water collected on the floor, on the vapor barrier, or in low spots over the soil. Unlike the steady dampness of ground humidity, it means water is entering the space faster than the space can shed it, through runoff and poor drainage, a high water table, or a leak.
Pooled water keeps the air saturated, so the beams, joists, and sill plate above stay damp, and sustained dampness is what decay fungi need. Saturated soil also loses bearing strength, and water held against a foundation wall presses on it from outside. Homeowners usually notice a musty smell, a soft spot in the floor, or higher humidity upstairs first.
The source determines the lasting fix. Sealing a crawl space with crawl space encapsulation services works only once the water reaching it is managed, so a free inspection first identifies where and how the water is entering, checks the framing for decay, and measures floor elevations before any repair is discussed.