Bollinger Bands draw a stretchy envelope around the price — an upper line and a lower line — set a certain distance above and below its recent average. The clever part is that this envelope is not fixed: the bands automatically widen when price gets wild and squeeze in tight when price goes quiet.
When price pushes all the way up to the top band, it is stretched high; when it sinks to the bottom band, it is stretched low. And when the two bands pinch in unusually tight together — a squeeze — it is telling you the market has gone remarkably calm and coiled. That quiet very often comes right before a big, sudden move bursts out in one direction or the other.