When the tide or the sea ebbs, its level gradually falls.

The ebb or the ebb tide is one of the regular periods, usually two per day, when the sea gradually falls to a lower level as the tide moves away from the land.

If someone's life, support, or feeling ebbs, it becomes weaker and gradually disappears.

If someone or something is at a low ebb or at their lowest ebb, they are not being very successful or profitable.

You can use ebb and flow to describe the way that something repeatedly increases and decreases or rises and falls.