what are coronal loops?

Loops in the Sun's atmosphere run between north and south poles of a magnetic field. Often the north and south poles are found in pairs of sunspots.

It is useful to think of the field in a single solar loop as being like that from an imaginary bar magnet underneath the surface

An example of how materials respond to magnetic fields is the pattern of iron filings around a bar magnet. The filings line up into loops going from the north pole to the south pole of the magnet. The same happens with plasma in loops on the Sun.

The magnetic field traps material, which can then only move along the loop, not across it.

Here's a close up movie of an active region as it moves with the Sun's rotation over several days. See how the active region changes with different hot loops lighting up at different times.