Uh... I've watched a video from @ScriptGuider on how to make a Custom Chat. I really don't understand very well a few parts of what he wrote, so I would appreciate an explanation of some bits of code that I'll indicate. As I want to learn on how to FilteringEnabled, I understand the structure in general on how it works, but there is parts that I don't understand, so I hope I get some more understanding of this after this post!

ServerChat > ServerScriptService

--\\ Services
local RepStorage = game:GetService'ReplicatedStorage'

--\\ Remotes
local ChatEvent = RepStorage:WaitForChild'ChatRemote'

--\\ Remote event
ChatEvent.OnServerEvent:connect(function(Client, Request, Value)
    Request = Request:lower()
    if Request == "chat" then
        ChatEvent:FireAllClients("chat", Value, Client.Name)
    end
end)

ClientChat > StarterGui

--\\ Services
local RepStorage = game:GetService'ReplicatedStorage'
local Players = game:GetService'Players'
local StarterGui = game:GetService'StarterGui'

--\\ Remotes
local ChatEvent = RepStorage:WaitForChild'ChatRemote'

--\\ Client stuff
local Client = Players.LocalPlayer
local Mouse = Client:GetMouse()

--\\ GUIs
local Chatbar = script.Parent
local ChatFrame = Chatbar.Parent

--\\ Chat messages
local Chats = {} -- Why is this Chat table empty?

--\\ Push messages
local function PushMessages(Size) -- How does this argument identify himself as the Size of the GUI?
    for i,v in pairs(Chats) do
        v.Position = v.Position - UDim2.new(0,0,0,Size)
        if v.Position.Y.Offset  < -ChatFrame.AbsoluteSize.Y then
            Chats[i] = nil
            v:Destroy()
    end
end

--\\ Create message
local function CreateMessage(Message, Speaker) -- What are the argument's utility here?
    local Label = Instance.new("TextLabel")
    Label.BackgroundTransparency = 1
    Label.TextXAlignment = "Left"
    Label.TextColor3 = Color3.new(1,1,1)
    Label.Font = "ArialBold"
    Label.FontSize = "Size18"
    Label.Size = UDim2.new(1, 0, 0, 25)
    Label.Position = UDim2.new(0, 0, 1, -25)
    Label.Text = tostring(Speaker) .." : " ..Message
    PushMessages(Label.Size.Y.Offset)
    Chats[#Chats+1] = Label -- What does this line do?
    Label.Parent = ChatFrame
end

Mouse.KeyDown:connect(function(Key)
    if Key:byte() == 47 then
        Chatbar:CaptureFocus() -- CaptureFocus() Is a function of the Mouse?
    end
end)

Chatbar.FocusLost:connect(function(Enter) -- Does the Enter argument identify himself as the Key:byte() that's equal to Enter?
    if Enter then
        if Chatbar.Text == '' then
            return
        end
        ChatEvent:FireServer("chat", Chatbar.Text)
        Chatbar.Text = ''
    end
end)

ChatEvent.OnClientEvent:connect(function(Request, Message, Speaker) -- What are this arguments for?
    Request = Request:lower()
    if Request == "chat" then
        CreateMessage(Message, Speaker)
    end
end)

StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Chat, false)

My questions about this are:

What does OnServerEvent function do?

Is it a communication between the Server and the Client?

What does OnClientEvent function do ?

Is it a communication between the Client and the Server?

I'll include commentary questionsinside the script.