The High Grab is a trick in The Legend of Zelda: Ocarina of Time. Specifically, it is any occasion in the game where Link is seen to suddenly grab onto a ledge, even though the game's graphics would lead you to believe he should have fallen short of it.
There are three major known High Grabs.
High Grab I, Fire Temple
This one isn't a secret strategy in any way since the game requires you to do it to finish the Fire Temple. Still, it's listed here because it demonstrates the principle. In the Fire Temple, on 2F, there is a semicircular room flooded with lava with a door at each end and a locked door near the middle. The room has suspended gratings for platforms across it, and if you start to cross the room in either direction, a wall of fire will appear behind you and move slowly across the room to chase you.
The locked door appears to be on a ledge which is much too high to reach just by jumping. However, there is no other way to reach it, and it CAN in fact be reached. Just jump out after measuring a reasonable run-up and Link can grab the edge and pull himself up, even though it looks for an instant like he's going to fall short.
High Grab II, Water Temple
In the main room of the Water Temple on 3F is a prominent ledge where a Triforce symbol can be seen. Playing the Ocarina of Time at this symbol raises the water level in the temple to 3F. Normally, the only way to reach it is to enter from the barred door visible behind it; the ledge appears to be too far away to reach by jumping from the 3F terrace. However, it is possible.
Stand at the corner of the central column of the temple, facing the ledge. Move a little to the right and backwards so the corner is slightly to your left. Face the ledge. Do NOT face what appears to be the nearest part of the ledge; it is impossible to grab this bit. The spot you are aiming for is very slightly to the left, maybe a third of the way along the edge. Run forwards and jump off the edge, hitting A to roll right as you reach the edge, and give yourself a minuscule amount of extra distance. Try again if necessary.
The High Grab II was discovered by "FierceDeityLink".
High Grab III, Water Temple
This is slightly more unusual in that jumping isn't required. En route towards the Boss Key you will arrive at a partially flooded room with Stingers inhabiting the water. There are some bombable walls and a pressure switch. When pressed, the switch floods the room completely, affording you access to the door to escape. You're supposed to bomb a few walls, pull a block around and push it to land on the pressure switch permanently, so you have time to swim over and leave the room at your leisure. This is unnecessary if you do the following:
Kill all the Stingers if you wish. Land on the pressure switch with your Iron Boots and stay there until the water level stops changing. Turn to face directly at the exit door and move to the front edge of the switch in preparation. Take off your Iron Boots and swim rapidly forwards, tapping B, towards the edge you're trying to reach. You will be delayed slightly by a bit of wall, don't worry, keep swimming. You will float up as the water level drops, and ultimately grab the edge you're aiming for as if by magic.