I have read the expression of 3D illustration a lot lately but I don’t know what it means… is it just a fancy way of saying 3D modelling?
You read it a lot because it’s become very trendy.
3D modeling generally means creating a digital model in 3D space.
3D illustration employs the look of 3D for illustrative purposes, creating a scene that gives the impression of 3D even though it’s actually displayed in 2D space. A designer might use 3D models to create that look, or might also fake it by drawing the scene in 2D with all the cues that make it look 3D.
Image searches will show you examples of both. If you really want to be impressed, search also for “hyper-realistic 3D drawing”.
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Thanks for the detailed reply 
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It used to be(!) that 2D illustrators would use “amazing airbrush techniques” to simulate a “3D appearance” to their finished artwork. These days, however, illustrators can also go the other way. Today, they can create an actual 3D model, most likely in Blender, then use the computer to render it in such a way that it appears to be “an illustration.”
The real experts in this new art-form can do it so well that you really can’t tell the difference. (Nor does it matter, anymore.)
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