An additional mob responding to the threat you represent is called an Add. It is frequently typed in Party Chat to inform party members that an uninvited mob has attacked you and that you need assistance. If you are a Healer or a mage class then assistance will usually be immediate but if you are a Damage Dealer then you are expected to fend for yourself until the mob currently being slain by the party has been defeated.
If you find that you frequently pull Adds, you should examine the skill level of any active skills you are using and also assess how close you are standing to nearby mobs. You may need to attack nearby mobs first or maneuver yourself to attack the particular mob you want to target without leaving yourself exposed to Adds. If mob proximity is not the problem then learn how to use Partial Strength Active skills.
A supportive mobs is a special form of Add.