I got 2 different lines of code, Can someone help me understand their difference?

At the first line you are accessing the TestService. At the 2nd line you are locating the TestService. Also you forgot to give the local variable a name.

local TestService = game:GetService("TestService")

(TestService doesn't exist so it should create a new one)

game:GetService("TestService") is used when someone tries to use TestService once (in this this case it's TestService, but it could be Workspace, RunService or even MarketplaceService).

Example 1: let's use the Workspace service and change the baseplate's color:

game:GetService("Workspace").Baseplate.BrickColor = BrickColor.new("Black")

There I only change the color of the baseplate once, so it's fine to use game:GetService("Workspace") since I only need to access the Workspace service once to change the baseplate's color.

local VAR = game:GetService("TestService") is used when you need to access the service frequently (to save memory and CPU usage). This also creates TestService.

Example 2: Let's do the same as Example 1, but have it change color every second for a minute:

local workspace = game:GetService("Workspace")
local part = workspace.Baseplate

for i=0, 60 do
    wait(1)
    part.BrickColor = BrickColor.Random()
end

There the baseplate's color changes frequently, so I store both the Workspace and part as a variable, to access them quicker. This also gives better performance since I don't have to find the Workspace service and the baseplate every second.