Building on the answer from ZargulTheWizard, it's likely you've started with low resolution geometry and done a "Subdivide" at some point to add extra geometry, but it hasn't added any extra topological detail. I'd say using a "Subdivision Surface" modifier is indeed the way to go, with a "crease" trick which I'll show in a moment.
First, let's look at the problem you currently have. I could guess you arrived at your current level of detail by doing something like the following steps:
(Click for larger version)
- (a) started with low-detail mesh.
- (b) selected inner and outer rings and used the "Subdivide" operation with maybe a step count of "3" -- though there are probably other ways the same result could've been achieved.
- (c) the mesh looks like it has a lot of extra detail now, but...
- (d) ...when viewed as a shaded model, it's clear the extra faces are just coplanar, and so they still just describe a flat surface instead of a smoothly curving surface.
Now, assuming you can go back to your original low-detail model, below is how you might re-do it differently. (NOTE: If you need a method to actually reverse the steps above, in order to get back to step (a) above, let me know).
- Go into Edit mode of your original low-detail model (i.e. select it, hit TAB). (NOTE: This method won't have the desired effect if you start with your current incorrect high-detail model, so other additional steps would be required first to revert it; let me know if that's the case).
- Go into Edge selection mode (i.e. hit 2).
- Select the inner and outer edge loops from both the top and the bottom (i.e. ALT+LMB for the first loop, then SHIFT+ALT+LMB for the remaining 3 edge loops.
- Go to the Sidebar (toggle its visibility with N key), click the "Item" tab, and set the "Mean Crease" to 1.00
- Now add a "Subdivision Surface" modifier (i.e. "subsurf"), and you'll hopefully see that it has increased the detailed smoothness of the round part, while keeping the hard corners and flatness of the top and bottom parts.
You can now optionally change the "Levels" of the subsurf modifier for even more detail/smoothness, and "Apply" it before exporting an STL to ensure that extra geometry exists for real.
If necessary, there are also ways to remove the extra unneeded lateral divisions and reduce your overall polygon count, but I doubt that will be necessary for a 3D print.