What do these tow lines of code mean?

Can someone explain these two lines of code please,

Line one

Code:

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T& operator[] (int i) {return at (i); }

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What is 'operator', and 'at'?

Line Two

Code:

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const T& operator[] (int i) const { return at(i);}

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What is going on here?

Thanks!

I am not sure if I can explain this good enough but it is operator overloading of [].
operator is a key word telling the compiler that what is following should be done when the "operator" is found. In this case [] but it could be /*-+ to. at is a function i is the argument; It makes it possible to do things like

CString one; //Object of CString class
CString two; //Object of CString class

one = one + two; //Use the overloaded operators = and + to do something with the CString objects.