The E-band was a range of electromagnetic frequencies. E-band emissions were sometimes emitted by collapsing protostars but were very difficult to localize and their occurrence in space was rather unusual.
When Romulans brainwashed Geordi La Forge in 2367, they sent him secret orders by means of E-band transmissions directly to his VISOR. These signals were delta-compressed on a frequency that is very similar to Human brainwave patterns. The Klingon ambassador Kell, secretly working for the Romulans used a small concealed E-band transmitter to send orders to La Forge. (TNG: "The Mind's Eye")