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[NiLSPACE]

When you use table.insert you basicly create an array.

local Table = {}
Table.insert(Table, "Test")
print(Table[1]) -- Result is "Test"
Table.insert(Table, "Test2")
print(Table[2]) -- Result is "Test2"

When you use your way:

local Table = {}
Table["Test"] = "Test"
print(Table["Test"]) -- Result is "Test"

You add a key. This is propably better then using table.insert because you already know the name of the key.
If you want to check if he's in the table you can use:

if (rank[Player:GetName()] == nil) then
 -- He's not in the table
end