The CharacterRemoving event fires right before a player’s character is removed, such as when the player is respawning.

This event can be used alongside the Player/CharacterAdded event, which fires when a player’s character spawns or respawns. For instance, if you would like print a message every time a player spawns and dies:

local Players = game:GetService("Players")

local function onCharacterSpawned(player)
	print(player.Name .. " is spawning")
end

local function onCharacterDespawned(player)
	print(player.Name .. " is despawning")
end

local function onPlayerAdded(player)
	player.CharacterAdded:Connect(function ()
		onCharacterDespawned(player)
	end)
	player.CharacterRemoving:Connect(function ()
		onCharacterDespawned(player)
	end)
end

Players.PlayerAdded:Connect(onPlayerAdded)

This event is only concerned with the Player/Character|Character of a Player. If you instead need to track when a player joins/leaves the game, use the events Players/PlayerAdded and Players/PlayerRemoving.