In practice
In a rising wedge price is still making higher highs, but each push gains less ground than the one before: momentum is fading. Traders watch for a close below the lower line as the signal that buyers have run out.
A falling wedge is the mirror image at the end of a decline. The start of the wedge is a common first target.
What to watch for
A wedge can keep narrowing for much longer than seems reasonable, and shorting a rising wedge before it breaks means fighting a trend that is still up.