Generally speaking, volume tends to be a leading indicator of price. Volume usually drops off before prices peak, and usually picks up before prices rise from a market bottom.

Low volume levels are characteristic of the indecision that typically accompanies consolidation periods. Low volume is often found at market bottom.

High volume levels are characteristic of  market tops and the beginning of new trends. Just prior to market bottoms, volume will often increase on panic driven selling.

Volume also helps determine the health of an existing trend. A healthy uptrend should have higher volume on the upward legs of the trend, and lower volume on the downward, corrective legs.
After a market top, it is common to have a sharp down day on very heavy volume. A healthy downtrend usually has a higher volume on the downward legs and lower volume on the upward, corrective legs.