Durable Water Repellant. Fietsbob already beat me to it.

Basically, it's a chemical thing that gives a surface just enough static charge (?) to made water bead up and roll off, instead of soaking in. A lot of jackets, rain pants, and other outdoor gear comes treated with some kind of DWR. It lasts about a season, with typical use and typical washing, then it's more or less worn out. If your rain rain gear has a surface that, well, works with DWR however that works, then applying more will make your stuff as good as new.

Get the spray on kind, not the wash in stuff.