From SmashWiki, the Super Smash Bros. wiki
The jab grab is a character-specific technique performable by Pikachu, Mario, Ness, and Luigi in Smash 64. It is performed by jabbing, and then using a grab instantly afterwards, nearly at the same time. It can only be performed by characters with a frame 2 jab. Additionally, performing the grab too late will just perform another jab instead. It is a simple form of shield pressure, though it can be used when connecting with any damageable object.
Level 8-9 computer opponents occasionally use this technique as well, more often with Mario and Pikachu.
This technique is highly useful in competitive play, and it is one of the key reasons why certain characters are considered better at pressuring, as not having this technique highly limits shield pressure.