When changing this setting to be enabled, in case of a death, your inventory will still be kept as to before you died, removing the factor to lose any items.
First, make sure that you have given yourself Admin rights and can use Admin commands in-game.
Login to your Minecraft server and type in the command "/gamerule keepInventory true".
The chat will then present a message saying "Gamerule keepInventory is now set to: true".
This means the gamerule is now active and no items will be lost in case of a death.
