Several reasons. Normal fetal heart rates are 120-160bpm & as low as 110 at term. If lower, there are reasons that can be significant so an obstetrician should be able to evaluate these or make a referral to a high risk pregnancy specialist.
Depends. There are many variables involved to answer this question appropriately. A slow heart rate can be normal very early on. Later a very slow rate can signify a heart block in the baby. In labor the rate can be ominous or not depending how slow and the pattern on the tracing.
Fetal death. Very slow can cause fetal demise. Your OB would tell you if it was slow enough to be serious.