(Text and picture adapted from http://soi.stanford.edu/press/ssu8-97/pmodes.html)
The Sun is ringing, like a bell struck by many tiny sand grains. The above pictures show three different ways (or "modes") to ring.
The Sun acts as a resonating cavity, exhibiting millions of oscillation modes or standing waves.
Most people are familiar with oscillating modes in one dimension: if you shake one end of a tight rope you can set up an oscillating wave pattern that vibrates the whole rope back and forth together. If you shake faster, stable patterns with two, three, or more oscillations along the rope can be established. In these overtone patterns are places in the rope that don't move, those are called nodes. The patterns can be described mathematically with sine waves of only certain wavelengths depending on the length of the rope.
In two dimensions, more modes of vibration are possible. Try putting a coffee mug on the counter when the garbage disposal is running, and you may be able to see a fairly complex two dimensional pattern on the surface with nodes lying along certain fixed curves..
In three dimensions modes can be described in terms of mathematical functions called spherical harmonics. Spherical harmonics have characteristic spatial patterns with nodes located both on the Sun's surface and radially within the Sun.
Each mode is sensitive to the physical conditions where the amplitudes of the mode are greatest. It is easy to see that the various modes overlap, but that each one is sensitive to different parts of the solar interior.
By combining a large number of modes one can build up a picture of what is happening throughout the solar interior, and in particular how varies the rotation rates of the Sun with latitude and depth.
...Too see the tiny sand grains which cause the Sun to resonate, click here...