These are brain waves in the frequency band 4 - 7(11) Hz.
Theta waves do not occur, under normal conditions, in a fully awake adult individual. However, they are normally present up to puberty.
Theta waves occur in the interface between sleep and waking and are considered to be related to the creational/creative process and emotionally charged fantasies.
However, theta activity can, with electrodes, be recorded in the septal region and in the hippocampus of an awake adult.