This is important to know: everyone has an "active" and "functional" pineal gland. The pineal gland's primary function is to produce melatonin, which helps to regulate the sleep-wake cycle. If your pineal gland "didn't work," you literally could not function as a human being. That is only one of its many functions. The one Satanists mainly refer to when talking about it is that of a "biological transducer."
The definition of a transducer is "an electronic device that converts energy from one form to another." Essentially, it converts energy into something we can understand and perceive. Actually, the pineal gland is essential to astral sight, as even if you have a completely open and functional third eye, without the pineal gland, it's like having a high-definition camera that is not plugged into a monitor; you can't see the feed.
It takes a lot of energy [compared to the normal non-spiritual human baseline] for the pineal gland to operate at a level where this ability functions as intended. When we meditate on the pineal gland, this is what we are working toward.