These bombz are directional explosive charges the size of manhole covers. They are used by Orks against vehicles and fortifications which krak stikkbombz just aren't heavy enough to break. Tankbusta bombz are used just like krak grenades to attack vehicles and bunkers. However Tankbusta bombz double the D6 roll for penetrating armour, giving them an armour penetration of 6+(D6x2).
Tankbusta Bombz
Source: Codex: Orks, p. 36