I do not try to explain FHR tracings to families and patients, unless I have a situation where repetitive lates or variables are becoming a problem, or a prolonged decel happens. If they press me, I gently explain it takes a lot of practice and training to interpret monitor strips and can't be learned in a five minute session from me. Their response to this will depend a lot on how well trust has been established.
If I'm the admitting nurse (or at the beginning of my shift if I'm not), I take time to explain that baby's heart rates will occasionally fluctuate in labor, and I might come in to ask her to reposition or place some oxygen. I tell them that 99% of the time this is our normal practice and does not mean an emergency is happening. This seems to reduce a lot of fear/anxiety if I do have to do those things.