Priority Linking, often shortened to Plinking or P-Linking, is a term used in certain fighting games to refer to an advanced technique with the purpose of making combos, especially difficult ones, easier to pull off. This mechanic is most often associated with Super Street Fighter IV and the subsequent updates.
Plinking Heavy Punch and Medium Punch. Note the result of inputing only two commands.
Description
Plinking is possible through a programming detail in Super Street Fighter IV based on exploiting the priority mechanic of the game. This "priority" mechanic simply gives certain button commands higher priority than others, thus the name. Heavier attack inputs have higher priority than lighter attacks, whereas kick inputs have higher priority over punch inputs of the same strength level (light, medium, or heavy). The priority order is as shown below:
< 
< 
< 
< Hard punch < Hard kick
By exploiting this mechanic, the player gains an extra frame to hit the next move in a combo, which potentially doubles the chance of landing one-frame links.
Technique
Plinking is done by quickly pressing two or more buttons exactly one frame apart (in priority descending order). This can be done by rolling one's fingers across two button, almost hitting them simultaneously. Because of input priority, the game makes first input come out either at the frame where the first button is pressed or at the next frame when second button is pressed.
It is easier to perform on arcade sticks than controllers.
Tutorials
P-Linking AKA Priority Linking
In-depth tutorial
SSF4 Plinking Slowed Down and explained
Quick tutorial