When an electric guitar or two electric guitars such as in the case of Lynard Skynard have distortion turned on and are playing the same note, normally the 3rd or 5th note of the scale at the exact same time but at different octives. This of course causes a goose bumpy sensation.
An example is in the chorus of more than a feeling. Boston uses alot of thunder chords. Also in Boys Are Back In Town, the repetitive guitar lick is a grouping of thundercords.