what does the function table.concat do?

The concat function in the table library takes a table of strings and concatenates or puts them together:

local strings = {"foo" , "goo" , "bye world"}
local combinedStrings = table.concat(strings)
print(combinedStrings) --> foogoobye world

As you noticed above, the words do not have any spaces between them. The concat function has another optional argument which is a string to insert between each value:

local combinedStrings = table.concat(strings , " ")
print(combinedStrings) --> foo goo bye world

Finally, the concat function has two other optional arguments that tells a range in the table to concatenate rather then the whole thing:

local lotsOfStrings = {"S1" , "S2" , "S3" , "S4" , "S5" , "S6" , "S7" , "S8"}
local combinedStrings = table.concat(lotsOfStrings , " string " , 3 , 6)
print(combinedStrings) --> S3 string S4 string S5 string S6 string