A yabby is a small crayfish that is found in creeks, rivers and billabongs in Australia. They can be caught and eaten in much the same way as larger crayfish.
Yeast is a very tiny fungal organism. Some species of yeast are used in baking and brewing. Yeast can make bread rise and become light and fluffy, and it can be used to make some wines and beers.
Yeast extract is used to make some very strongly flavoured spreads like Vegemite.
Yoghurt is made from fermenting milk. Yoghurt is very sharp and tangy on its own. Modern yoghurt often has fruit and sugar added. Yoghurt is nice to eat on its own, with fruit or breakfast cereal, or frozen as a kind of icecream.
The yolk of an egg is the yellow part inside the egg. The clear or yellowy-white part is called the white of the egg. It goes white when you cook it, and the yolk stays yellow but goes crumbly.