I recently put my rooted cuttings outside. This has appeaared. What is it? Will it kill them?

It looks like sun burn to me. If so, they'll be fine; they just needed to be acclimated to direct sun.

it's normal to have some burns on the leaves , even losing them because you got them out in the sun and planted in the ground, new leaves will emerge and all will be good :)

Too much sun, too fast.