Why "thin enough on screen" is not the same as printable
A CAD model can show any wall thickness you draw, including one thinner than your printer can physically extrude. An FDM nozzle deposits material in fixed-width lines, and a slicer builds a wall from a whole number of those lines. A feature drawn between two multiples of that line width does not get a proportionally thinner wall; instead the slicer typically rounds down to the nearest wall count it can fit, silently changing your design's actual thickness in the sliced result. This check compares your requested thickness against your nozzle and wall-count assumptions before you find that out on the printed part.
How the comparison works
Line width is estimated from your nozzle diameter (a common default is about 1.125 times the nozzle, matching most slicers' default extrusion width) unless you enter your own value. The requested thickness is compared against a single line width and against the full width needed for your chosen wall count. A thickness below one line width will likely not print at all; a thickness between one line and the full wall-count width usually means the slicer reduces the wall count at that feature rather than printing what you asked for.
Worked example
A 0.30 mm wall requested at a 0.40 mm nozzle (about 0.45 mm assumed line width) with 2 walls specified needs about 0.90 mm to fit both walls cleanly. Since 0.30 mm is thinner than even a single line, the tool flags this as a thin-feature concern and asks you to either thicken the feature or move to a smaller nozzle, rather than letting the slicer quietly decide what to do with it.
Limitations
This is a geometric comparison against the nozzle and line-width values you entered, not a simulation of your specific slicer's wall-generation algorithm, which can vary between slicers and versions. Always confirm a thin or critical feature in your slicer's layer preview, and print a calibration piece before committing a full part that depends on an exact wall thickness.