String Pattern confusion

[Judgy_Oreo (phoenixwhitefire)]

@7z99

I’ve tried a few code samples from a few websites upon doing some research on substrings but I genuinely have no idea how or why they work.

local s = 'aaaaa' 
print(s:find('()aa()'))
--> 1 2 1 3???

string:find just returns where the matched characters start and end in the inputted string