Fry are baby fish; raising them needs the right tiny foods, clean stable water and (usually) protection from adults, whether they arrive planned or as a surprise livebearer brood.
Newly hatched fry often need very small foods (like infusoria, powdered food or baby brine shrimp) and gentle filtration that will not suck them in — a sponge filter is ideal. Frequent small water changes keep them growing.
A surprise batch of livebearer fry is a common first breeding experience; deciding in advance whether you can house or rehome them is part of responsible keeping.