"Abnormal bleeding" is any type of bleeding that is not typical or "normal" for most women, most of the time.
"Normal" has a wide range of definitions, but typically women "normally" experience bleeding that is 3-5 days long, once per cycle, and this bleeding has some heavy days, as well as some light days. Normal bleeding can be thicker, have clumps, or be thin. The color can be any combination of red, pink or brown.
If the bleeding you are experiencing is not within this normal range, then this is when you know you may have "abnormal" or "irregular' bleeding. Your doctor is right: if you bleed heavily enough to soak through more than one pad or tampon in an hour, you need to call her immediately.