The term “half and half” shows up very frequently in cooking and baking recipes in the US, but it is not particularly well known outside of the US (or the term refers to something completely different). When it appears in a recipe, it sometimes generates a little confusion as to what it is. “Half and half” is a mixture of half heavy cream and half whole milk that is commonly found in the US. Heavy cream has a fat percentage of about 35% and that concentration of fat is what helps stabilize it when you whip it into whipped cream. Half and half has a fat percentage around 12% and, although much thicker and creamier than milk, it will not whip up into fluffy whipped cream like heavy cream will.
The rich, creamy consistency of half and half is ideal for adding to black coffee to lighten it up without watering it down and for adding to soups to enrich them. It can be used in place of milk in many recipes, such as cakes and quickbreads, and will give baked goods an even finer, more tender crumb than they would typically have (although it is worth noting that the extra fat can create a product that is too rich and even slightly greasy occasionally). My favorite application for half and half is in ice cream, where may recipes will call for both heavy cream and milk. Using half and half saves me a step when mixing up my ice cream base, it also saves me a step when writing the recipe because I can call for one convenient ingredient rather than two separate ones.
You can substitute for half and half by mixing half whole milk and half heavy cream. IF you live in a county where “light cream” is widely available, know that it has a slightly higher fat content than half and half and should be mixed with approximately one third whole milk to make a substitute for half and half.