Blast protection is an enchantment that can be applied on armor, this enchantment will reduce the amount of damage received by the player from explosions and fireworks. Its maximum level is 4 and can be applied on all armor pieces: helmet, chest plate, legging, boots, and turtle shell.
Blast protection on a diamond chest plate (via screenshot)
Blast protection is an important and quite useful enchantment as it reduced explosion damage and even knockback. The formula by which damage is reduced is 8% per level of blast protection which means a maximum of 32% reduction of explosion damage can be achieved per armor piece with blast protection level 4.
Now theoretically the damage reduction can be completely nullified at level 13 because adding up the protection from each armor piece adds up to 100%, but the developers thought of this and have capped the reduction at 80%. This enchantment also reduces explosion knockback by 15% per level, but this number is rounded down to the nearest integer (100%) thus making it noticeable if the player has blast protection level IV.
To enchant your armor with blast protection you will need an enchanting table or anvil. Enchanting tables will give a random list of enchantments that can be applied to the armor but their strength can be increased by placing bookshelves around the enchantment table.
Enchantment list on an enchantment table (via screenshot)
The other way is to find an enchanted book with the blast protection enchantment on it, now with the help of an anvil players can apply the enchantment to their desired piece of equipment. Both processes require the player to have a high enough experience level.
Using an anvil to apply blast protection on a diamond chest plate (via screenshot)
Blast protection is a great enchantment to have especially if you plan on exploring caves where creepers can kill you easily, so not dying in an instant is nice. Other than that the knockback can be pretty handy when fighting hoglins or iron golems. But getting it to max level can be challenging so the best way is to use the enchantment table and keep refreshing the list.
A. Blast protection will protect the player from mobs that do explosive dmg and launch explosive projectiles.
A. The maximum level of blast protection is level 4.
A. Yes, they will reduce explosion damage from fireworks.