Can anyone explain character?

A character is the model of the player located in Workspace and can experience physical objects and jump. It contains the humanoid and body parts. The player has the backpack, leaderstats, and is the one connected to the server. You can get the character in a variety of ways. Here are some:

-- For localscript
local player = game.Players.LocalPlayer
local character = player.Character or player.CharacterAdded:Wait()
-- either finds the character or waits for it to load in

For script

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(character)
		wait(2) -- cooldown for character to load in
	end)
end)

For more information go here: https://developer.roblox.com/en-us/api-reference/property/Player/Character/index.html

To get every character:

local players = game:GetService("Players")
for _, player in pairs(players:GetChildren()) do
	if player:IsA("Player") then
		-- unneeded line
		local char = player.Character or player.CharacterAdded:Wait()
	end
end