This is called a double whole note or breve.
Whereas a whole note is equivalent in duration to four quarter notes, a double whole note is equivalent to eight quarter notes.
We see it often in transcriptions of older music, where the half note is used as the beat value instead of the quarter note. Consider, for instance, this example from Palestrina; also make sure to check out the breve rest!
Note that some modern notation systems only write one bar on either side of the note, but it's still a double whole note.