Merging is a skill from The Legend of Zelda: A Link Between Worlds. Enabled by wearing Ravio's Bracelet, it allows Link to merge with certain walls at will, rendering him similar to a cave painting. This allows Link to access areas he would not otherwise be able to, and pass through special cracks to travel between Hyrule and Lorule.
While Link is merged with a wall, the Energy Gauge is gradually depleted. Once the gauge reaches zero, Link will be forcibly returned to his normal state, causing him to fall and take damage if he is situated directly over a bottomless pit and the like. Obstacles that limit Link's movements include ledges, windows, torches, and formations of rough rock.
Yuga is also able to merge with walls at will. He puts this skill to use at several points, most notably during his first encounter with Link in the Sanctuary, causing a charging Link to crash straight into the wall. Later, Yuga attempts to stop Link's attempts to thwart his plans by imprisoning him inside a wall. At this point, Ravio's Bracelet reacts, allowing Link to pull himself out of the wall, and harness this power for his own use.