The way it works is that she becomes full of eggs from time to time. If there is a male in the tank, they may then spawn: she lays her eggs and he squirts his sperm over them as they are laid. The fertilised eggs take a couple of days to hatch, the fry then float around eating their egg yolk for a few days until they have finished it, then they become free-swimming and have to find food for themselves. Danio parents pay no attention to their offspring, but may well eat the eggs, they have no sense that these are their babies.
If there is no male, and no spawning process, then the eggs cannot be fertilised- so no baby.
Other egglaying fish, such as cichlids, look after their babies after they are born. This can create trouble in a tank as they then try to defend them against all comers.
With livebearers (guppies, mollies etc), it's more like mammals. They mate- the male inserts sperm into her- the egg is hatched inside the mother and free-swimming babies come out. Some livebearers even have a primitive umbilical cord, so that the fry can actually get food from the mother (not just from the egg) while it is still inside her. This means that the fry is bigger and better able to cope when it finally does come out. So these fish tend to have fewer babies as more will survive. Livebearers can be pregnant in the sense that a female carries fry for several weeks after the mating; so even if you buy a single female and no male, you can still get babies (this clearly can't happen with egglayers). Many livebearers can store sperm and use it for a succession of pregnancies.
