Imagine, suggests Maldacena, an electron. Like the Earth, the electron exerts some gravitational pull, which increases as you approach it. But conventional physics describes the electron as an infinitely small point-like object. So, on paper, there is no limit to how close you could get to the electron, and calculations might show its gravitational pull soaring to infinity, which makes no sense. Its gravitational field is smaller than the nebulous space where it is expected to exist.
String theory helps in a number of ways.
It considers all the particles of physics to be string-like bands of a very small, but definite size. All the strings are the same, except they vibrate in different ways depending on what species of particle they are. So an electron is a tiny string that vibrates in a way unique to electrons. When physicists know how big the electron is, even if they still can't say exactly where it is, they can start to make calculations about its gravitational field. Shortly after physicists first started hypothesizing about string-like particles in the 1970s, they noticed that one of the vibration modes corresponded nicely with the characteristics of the graviton.