Do 

scientists really know what gravity is?

Yes, it is true.

We can calculate it with an incredible degree of precision. We can describe its effect all but completely, but we do not have a theory on what gravity is, not yet - despite the popular saying.

Typically we describe gravity as a fundamental force of nature (one of four), but physicists argue it may be an emergent phenomenon instead. Evidence support both.

I’d say the following points help illustrate what we know and do not know about gravity:

1. “Newton’s Universal Law of Gravitation” (1686) - offers no explanation to what it is. It is a law that describes how it works, not what it is.
2. “Einstein’s General Theory of Relativity” (1915) - tells us that planets and stars (any matter) bend the fabric of space, and thereby produce gravity as an effect of this fabric bending - yet we have no theory to tell us what this fabric is made from, or even whats its fundamental properties are.
3. The nearest to a “theory” of gravity we have, is the “General Theory of Relativity,” and even this doesn't really describe what it is.
4. We have made up a term to cover for our lack of knowing why gravity cannot hold galaxies together. Our current understanding of gravity tells us galaxies would fly apart, especially at their outer rims. They do not. They should. Ergo, something-something-gravity keeps them from doing this. We call this ”dark matter” (“dark gravity” would be a better term?).
5. We know gravity is not always uniform, even when conditions are unchanged. We believe gravity is the same, and what is measured today will measure the same tomorrow. It is the consensus. However, there is some evidence suggesting, this may not be true. Inconsistent gravitational anomalies have been recorded when we slingshoot spacecraft around Earth - more specifically, data from Galileo &, NEAR Shoemaker show irregular gravitational deviations that should not be. We do not know why. [EDIT: A solution was found in 2013, ref; Anomalous Earth flybys of spacecraft - thank you to Eric Platt for pointing it out in the comments]
6. “Newton’s Law of Universal Gravitation” is not universal. It fails near very strong gravitational fields, such as black holes, and it is assumed to be false when the gravitational fields are very weak, like amongst atoms. Moreover, we have no solution. Indeed “The General Theory of Relativity” bests “Newton’s Law of Universal Gravitation” when gravity is very strong, but it too fails amongst atoms.
7. We have not tested gravity as well as the other forces. We assume gravity is among all matter, but we have never successfully tested this thoroughly. By contrast, we have with the electromagnetic force, but not with gravity. We know our equations begin to fail slightly when two objects are already one meter apart.

So, indeed; we do not know what gravity actually is. We have ideas, and we can calculate it very well, and even calculate our mistakes (or lack of knowing) very well. But so far, the theory that nails it, elude us.