In literature, it would mean a story set in a naturalistic world (Earth) with fantasy elements. High fantasy stories are set entirely in a fantastical world, like LOTR, etc.
In video games, the terms "high/low fantasy" have come to have entirely different meanings. The term "high fantasy" for video games has been associated with lots of JRPGs with extremely fantastical settings. I haven't heard the term "low fantasy" used to describe a video game, but I would take it to mean "the opposite of high fantasy games" - in other words, the opposite of JRPGs, or a game with slightly more realistic characters and settings, as opposed to games with extremely bizarre characters and art direction.
http://kotaku.com/5194643/high-or-low-fantasy-in-dragon-age