We spent quite a bit of time discussing how energy shows up in earlier physics, so how is it carried here? Strings can carry energy in essentially three ways: by being stretched (like the energy stored in a rubber band), by moving as a whole (like a point particle), and by vibrating (like a guitar string).
Let's go over the new parts in a bit more detail. The effective mass of a string depends on its length: stretching it costs energy, and its "natural" mass is zero. As for vibrations, they will travel around the string in one direction or the other (we arbitrarily call them "left-moving" and "right-moving" to distinguish the two). But because the strings are so small, those vibrations can't be seen directly: they would look like a form of internal energy to us. You may already see where I'm going with this: a string's vibrational energy will look like mass to us. But first, we'll add quantum mechanics to the mix.
(The vibration modes shown here look like the quantum particle on a circle, but don't be fooled! These are purely classical motions at this stage.)
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