A chemical pregnancy is another word for a very early miscarriage or spontaneous abortion which aborts prior to seeing the fetus. A chemical pregnancy is a pregnancy that has been confirmed by a pregnancy test (either blood or urine), but there are no other significant clinical signs of the pregnancy and there is nothing seen on ultrasound. The urine pregnancy test is often faintly positive or blood hCg levels are very low and don't increase appropriately.
Although the blood test can detect a very low hCg level that indicates pregnancy a few days before a period, two to three days later, a period may start. Without the test, most women would never have known they were briefly pregnant.
A chemical pregnancy happens much more frequently than most people expect, probably up to 30-40% of all pregnancies end up in a chemical pregnancy, and without early testing, most women would never have known about it. If the pregnancy develops for several more weeks, a sac forms, but without an embryo. That's called a 'blighted ovum'.