A gaining stream has water seeping into it from the ground.
Streams Can Contain Groudwater
This diagram is a very general schematic of how groundwater contributes water into surface water (streams, rivers, and lakes). In this case, this is a "gaining stream", which generally gains water from the ground. Other streams are "losing streams", which lose water from the streambed out into the ground. Rivers can be gaining and losing at different locations; they can be gaining one time of the year and losing in another time of year. And, as the yellow arrow shows, even a gaining stream will be losing some water, and the other way around. In this diagram, though, we're showing a gaining stream where groundwater is entering the streambed.