Frog spawn is the name given to the clump of eggs that are laid by frogs. Frogs will generally mate in the Spring depending on the weather with the female laying eggs which the male will then fertilise. It is estimated that the average female frog can lay up t0 4,000 eggs at one go although many of these eggs will not survive to turn into frogs.
The eggs here are laid in water at which point they will sink to the bottom of the water to start off with. The end result is a clump of eggs which are all surrounded by a type of jelly material. This material will protect the developing baby frogs as they grow and help ensure the best available conditions. For example, the jelly around the eggs will swell up in water over time so that it kind of acts like a cushion and it brings the developing frogs on top of the water as it starts to float so that the eggs catch all available sunlight to keep them warm.
After a period of a few weeks the baby frogs (i.e. tadpoles) will come out of the frogspawn. They will then eat the frogspawn that is left over for the first few days after they emerge until they move on to other foods.