'Ground Under Repair' (also known as 'Grounds Under Repair' 'Abnormal Ground Conditions', or GUR) is a rule in golf that allows you to move your ball if it settles in an area deemed as abnormal ground conditions, whether under repair, under environmental protection, or nature's imposition on the grounds.
Specific rules for the nearest point of relief for the ball is noted here.
Areas considered to be GUR include newly planted turf, well-used turf awaiting regrowth, standing water not deemed a water hazard and new tree seedlings. These will usually be marked by a white line and may include a sign, or have some other mode of signifier, such as stakes and yellow, blue or white tape. Bundles of fallen branches or piles of leaves that groundskeepers have collected are also considered GUR, but are seldom marked. Many courses are often returfed, using rolled out carpets of turf, and for the matter of GUR, if a ball lands between the seams of the cut turf the ball may also be moved. Some clubs consider hoofprints and rabbit holes to fall under this rule.
Golf clubs by their very existence alter the landscape they are built on, and in many cases are invasive and detrimental to local plant species. A good amount of golf clubs have recognised this, and when adjacent to areas of environmental interest, these boundaries are also considered GUR.