A Bob-omb Buddy is a friendly variant of a Bob-omb, an enemy that typically attacks Mario on sight. Bob-omb Buddies function as NPCs and often provide context to the setting they are in.
In vanilla Super Mario 64, Bob-omb Buddies have bright pink bodies (or red, depending on your eye), as opposed to the black bodies that enemy Bob-ombs have. This makes it much easier to distinguish them from hostile Bob-ombs. Bob-omb Buddies can also be distinguished by their inability to move- due to limitations of the N64, NPCs that can be interacted with cannot move about. It is possible for enemies to have this limitation as well (for example, the enemy Bob-omb next to a cannon near the top of Bob-omb Battlefield's mountain), though very rare.
Bob-omb Buddies are a popular choice of side character in SM64 hacks. They are very often retextured to fit their new context, or simply for style.