Dash is a move in all of the Marvel vs. Capcom games where the character dashes. To perform it the player inputs left or right twice or presses all of the punch buttons.
Then there is dashing, which is performed via 2 attack buttons or double tapping either left or right. Dashing is a fast method of starting a battle if your opponent is either doing something unsafe or not doing anything at all. Whilst dashing is used a lot throughout the fight, it is quite unsafe as you cannot block while dashing. The solution is to crouch which cancels the dash allowing you to crouch block or dash again. (This is not available to some characters though). There is a technique known as Wave-Dashing which involves pressing , , , , , , etc. This method is very useful for covering ground in a very fast time. Characters with short or slow dashes can utilise this to get up and close with the character. This allows characters like Haggar to close in on the enemy if they are far away.
Another method of this is known as P-Link Dashing. This involves pressing , , , , , , etc. where the  is different from  such as LM, H. Plink dashing is known as Priority link dashing where one action has a higher priority than the other action, so in this case, dashing is higher priority than the attack. The dash can be cancelled into an attack, but when the dash button is pressed 1 frame after the attack button, the dash is given priority and the character will dash. However, this technique is very hard as you need to press the attack button 1 frame after the dash. That's 1/60th of a second. This technique is very strong as you can use that normal to your advantage. If you use  as the attack, you can throw them as you are holding +whilst being very close to them. If you use  as the attack, you can catch them trying to punish you as you dash in with a fast .
Characters with 8 way dashes can use Tri-Dashing. This involves jumping forward into the air and then dashing down-forward, creating a triangle movement. This method is only best when in the sky with Flight Mode. Other than that, it is not very useful. Characters like Doctor Doom need it as he cannot wavedash nor plink dash, his dash will put him airborne a bit. Tridashing is also performed via a superjump and then dashing down forward, creating more of a steep triangle movement. This will leave you airborne however, it allows you to perform an aerial attack which hits high. This is vital for mixups which is important in a game like Marvel vs. Capcom.