The i value runs from 1 and n (in this example) and you substitute whatever i value you're on into the part to the right of the symbol.
The symbol is a way of simplifying a long addition problem.
Lets do an example using the formulas you've been given.
Lets say n=5. This means that there will be 5 values of to consider.
Let the first one be called , the second etc up to .
Then what does is we start at i=1 and add together all values from up to which in this case is 5.
So we get .