When something ebbs, it is declining, falling, or flowing away. The best time to look for sea creatures in tidal pools is when the tide is on the ebb — meaning it has receded from the shore.
Ebb is often used in the phrase, "ebb and flow," referring to the cyclical changing of the tides from low to high and back to low again. This sense of cyclical change can also be applied to other things. If you want to make money investing, you have to weather the ebb and flow of the stock market.