What it means
Price can walk a band in a strong trend, so touches aren't automatic signals.
Example
SMA 100 and standard deviation 1.5: the upper band is 103.
In practice
The bands widen when the market is volatile and narrow when it is quiet, so they show at a glance whether price is stretched compared with its recent behaviour. A squeeze, with the bands unusually tight, tells traders to prepare for a larger move without saying which way.
In a range, moves to the outer bands often fade back to the middle line. In a trend, price holding near one band shows strength.
What to watch for
Selling every touch of the upper band or buying every touch of the lower one fails badly in trends. Work out first whether the market is ranging or trending, then decide what a touch means.