An implicit wait is a global timeout applied to every element lookup, and its default is zero. An explicit wait polls for one specific condition in one specific place, and it is the better choice because you name the exact state you need. A fluent wait is an explicit wait that you can tune with a polling interval, ignored exceptions, timeout and message. Selenium's docs warn against mixing implicit and explicit waits because it can cause unpredictable timeouts, and against fixed sleeps, which are either too short or needlessly slow. Race conditions between the browser and the script are described there as a primary cause of flaky tests.

Sources: Selenium waiting strategies