Expo changes the 'curve' of a control surface.

At "0 expo", which is to say no expo settings, move the stick halfway, the corresponding control surface (say, elevator) will also move 50% of its total travel.

Adding positive expo (usually..there can be exceptions) will 'slow it down'. moving the stick, say, halfway, will result in, for example, only 40% of the elevator's travel. in essence, it "slows down" the responsiveness of the surfaces, and thus the plane.

Negative expo has the opposite effect....a 50% stick movement might move the surface, say, 60% of its travel...making the airplane more responsive or 'twitchier".

There is, of course, a corresponding "spike" or 'flattening" of the curve at the end. If positive expo has the elevator moving only 30% of its travel for 50% of the stick movement, then the remaining 50% of the stick movement will have to produce the last 70% of the eleveator travel...so, near max stick, the surface becomes "hyper" responsive, as it were.

Exactly how much expo translates to how much movement dampening (or increase) depends greatly on the radio.

*EDIT* Sorry, Kat...i gotta type faster.
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Must agree with posters above...no reason not to throw some expo in. Saying 'don't mix things until you can fly without them" is, imo, like saying "Don't fly a trainer with dihedral until you can fly a sport plane without it". Ue the tools given to you to make your experience enjoyable and successful, imo.