Put simply, if the garment measures larger than the body, it has positive ease. If the garment is smaller than the body, it has negative ease. If the measurements of the garment and the body are the same, the garment has no ease.
Consider the two sweaters pictured below. Both are a size small, but the red sweater is much larger than the black sweater (which happens to be our Dolled Up Blouse).
Both sweaters fit The Animator's Wife perfectly well, but in different ways, and that's where ease comes in.
The red sweater was designed with positive ease in mind. It has a relaxed, baggy fit that looks 80s-exercise-chic when worn. By contrast, the black sweater was designed with negative ease in mind. The crocheted fabric stretches over the body when worn for a form-fitting look.
What does this mean for your own crochet garments?
It's usually not a big deal to make a larger size if you would prefer a more relaxed fit, but making the garment more formfitting can get a bit tricky. You'll want to make sure the chosen yarn and stitch pattern are stretchy enough that you'll still be able to get the garment on if you make it in a smaller size. This is where the gauge swatch comes in. You are making gauge swatches, right?
Note how when unstretched, the space between the two pins on the black sweater measures about 3.5 inches.
Without removing the pins, we stretched the fabric of the sweater, and now the space between the pins measures almost 6 inches.
This means the garment can stretch to almost twice its size to accommodate the body within it, making this yarn/stitch pattern combo a great candidate for making a garment smaller than your body measurements. Feel free to make this garment a size smaller than indicated in the pattern (assuming that the pattern was not already designed with negative ease in mind).
On the other hand, if you find that the yarn/stitch pattern combo called for in the pattern stretches very little or not at all (as is usually the case with solid single crochet), you'd probably be better off making your regular size, or else risk making a garment that is too small to wear.