Elevated heartbeat for prolonged time in my unborn baby. they are talking about taking it. whats going on?

Depends.: Many etiologies to persistent fetal tachycardia (i.e. >160 beats per minute for >10 minutes): infection, fever, acidosis, structural cardiac defects, true tachyarrhythmias. If your obstetrician has consulted a maternal-fetal medicine specialist and they recommend delivery, go for it. It is better to deliver the baby a little early than to risk cardiac failure and anasarca.

Simple: It means baby is under stress & they won't be sure what the cause is until they get baby out. Since it could be something bad or not so bad, they have only a little wiggle room before they decide to act.