What is the difference between:

"h" + "a" * 10

and

("h" + "a") * 10

and why are they different?

"h" + "a" * 10 results in "haaaaaaaaaa"
It concatonates the "h" and the "a" once, while adding the "a" ten times, as you've instructed.
("ha" + "a") * 10 results in "hahahahahahahahahaha"
It concatonates the "h" and the "a" into a string ten times, since the parentheses indicate the * 10 should apply to everything within.

Interestingly, while experimenting with this, I learned that in order for multiplication to work on a string, you have to start your line of code with a string. If you start with a fixnum, it will result in an error. However, if you start with a string, you can add any amount of math behind it, and it will work. For example, 10 * 10 * ("I will not" + " swallow my Ruby.") results in an error, but ("I will not" + " swallow my Ruby.") * 10 * 10 correctly displays the entire string 100 times.