A No. 77 hand grenade, is a smoke grenade, and it contained Phosphorus in its main body.
Phosphorus is a chemical that once it is exposed to air, it instantly catches fire and it burns / set fires to or burns through everything that it touches.
The only way to extinguish it is to stop all the air getting to it, eg to be totally under water.
This hand grenade is fitted with a No. 247 Allways fuze.
It would have been used to generate a smoke screen to allow Auxiliers to escape, or against enemy gun positions / pillboxes - vehicles and aircraft, fuel and ammunition dumps, etc.
To throw this grenade. You unscrewed and remove the Bakelite cap, but the cloth tape must stay in place. Then after you have thrown this hand grenade, the cloth tape which has a weight on the end of it, starts to unwind, and then pulls out the safety pin (the cloth tape and safety pin fall completely away from the grenade)now the grenade is armed, and when it hits the floor, or something else, the steel ball in the Allways fuze, pushes the firing pin forward to set the grenade off.
This fuze is called a Allways fuze, as it works at any angle - position.