Meaning, the pupils of the eyes get really big. There can be quite a few reasons for this.

1. The subject is looking at something desirable. This always includes an attractive member of the opposite sex. In Women, it also includes babies. In Men, it also includes landscapes, to a lesser extent, possibly as a vestigial remnant of hunting behaviors. This is why it's a good idea to go shopping for cars while wearing sunglasses. Otherwise, some salespeople will be able to read you much easier.
2. The subject is taking certain types of drugs. Other drugs can also contract the pupils.
3. If one pupil is large and the other is small, the subject may be brain damaged, and should receive immediate medical attention.

Dilated pupils are inherently attractive. In a psychology experiment, several men were shown two drawings of a woman that were identical in every respect other than the size of her pupils. Most men found the drawing with larger pupils the more attractive of the two pieces, although many didn't conciously notice the pupils, and couldn't explain why.