Layering involves using different products having the same fragrance to sustain the scent through the day. Think of it as a relay race where one product hands off the aromatic baton to the next. For example, you might start with an eau de toilette in the morning, smooth on some lotion in the afternoon, and finish with a dusting of powder for an evening out. Or start with a scented moisturizer and mist on perfume later in the day. You could add a few drops of scented bath oil to a warm bath, or apply to pulse points while the skin is still damp after showering.
The beauty of layering is that it lets you vary the concentration of the fragrance according to the situation. With a little experimentation, you can find a combination that gets you noticed but doesn't overwhelm anyone -- or waste expensive perfume. It's also a good excuse for a leisurely soak in the tub.