fruit trees

If one hits when the flowers are appearing, then yes.

Frost will affest the flowers as said, but leaf buds are less likely to be damaged. It also dpends on the level of frost too. We get pretty low temps here and the apples do not suffer too much, but the Pears, which open earlier are often hit. Cold wind is more likely to damage than actual frost, unless it really goes down into the well minus temps.
Sadly there is little one can do, unless the trees are small, in which case drape it with fleece or old net curtains over night. Or you could do like the Orchard folks and light a smoky fire so that the smoke drifts through the trees!

thank you i was concerned they are small trees and i will cover them as this is my 2nd year and dont want to loose any thanks once again bigalfie.