The crush system in Tekken 7, allows certain moves to make the character not get hit by high or low attacks, called high crush and low crush respectively. It works by the move causing the character to enter crouch or jump after a number of frames from the startup of the move.
HIGH CRUSH
High crushing moves will evade highs by causing the character to crouch during the move. This is might be referred to in frame data as tc (technically crouching) or cs (crouching state).
The generic d4 is an example of a high crushing move. It connects on the 12th frame after startup, however will make the character enter a crouching state on the 4th frame.
An in-game example of this would be Claudio 2,1 on block (-6) into d4 (cs4) would beat all highs including the generic standing jab.
LOW CRUSH
Low crushing moves will evade lows by causing the character to jump during the move. This is might be referred to in frame data as tj (technically jumping) or js (jumping state).
EVASION VS. CRUSHING
Due to the nature of Tekken's 3D hit detection, it's entirely possible for moves that are not actual tech crouches to still evade high attacks. Notorious examples of this sort of move include Lars' U/F+3 "Lightning Screw", Claudio's U/F+4 hopkick, and Paul's D/F+2 launcher.
This can be confusing, as a large portion of the player base simply uses the term "High Crush" to refer to any move that might slip under a high, regardless of whether the move is an actual tech crouch, or simply lower's the player's hitbox. However, in practice the distinction between the two moves can be incredibly important.
Certain high-evading moves, such as a Paul D/F+2, may be used to reliably defeat certain highs such as jabs, but will still get counter-hit by high attacks with larger-than-average hitboxes, such as an Electric Wind God Fist from a Mishima character. Paul's D/B+4 however, is a true tech crouch and will never get hit by an Electric Wind God Fist, or any other high attack, given that the tech crouch frames are active while Paul is attacked.
This is also extremely important for the game's more grappling-oriented characters, such as King, who have different sets of throws to use based on whether you think your opponent will be standing or crouching.
If you're playing as King and fighting Dragunov, it's important to know that even though Dragunov's B+2 tends to slip under jabs, it is not a tech crouch and is still vulnerable to your normal throws during its startup frames. Dragunov's D+2, however, is a true tech crouch that is immune to normal high throws, and can only be hit by a crouch throw during its startup frames.
"MID CRUSH"
Another term you may hear is that certain stances or moves, such as Xiaoyu's Art of Phoenix (AOP) stance, or Eddy's Relax stance, can "crush mids". This is something of a misnomer, there are no stances or moves in Tekken that will crush all mid attacks in the same way that a Low Crush will crush lows or a High Crush will crush highs. Stances like Xiaoyu's AOP will certainly lower her hitbox enough that certain higher-hitting mid attacks (for example, Bryan's F+3) will miss Xiaoyu completely. However, certain mid attacks, like a Jin B+2, 1, are low enough to reliably hit Xiaoyu even while she is low to the ground in AOP.
It's important to remember that due to the nature of Tekken's 3D collision detection system, the simple breakdown of Low, Middle, and High attacks do not always tell the full story of how characters interact with one another. However, the sure-fire reliable evasion of true Low Crushes and High Crushes can make them some of the most important parts of your arsenal.