Lemon curd is an egg and lemon based spread/sauce. It’s typically made by combining lemon juice, sugar, butter, and warming whipped eggs into the mixture over a double boiler. (Standard custard approach)

I’ve included a recipe from Alton Brown. Lemon Curd Recipe

There are many uses for lemon curd. I make a tart as described in one of my posts. A basic shortbread crust baked off. Spread fruit preserves, usually raspberry, over the crust. Pour the lemon curd, slightly cooled over the top. Chilled for a few hours, this is a nice bright summery dessert. It’s also heavy enough to stand up in the winter. Other uses: Spread over pound cake or a dense lemon cake. Spread onto toast or English muffins.

I hope that helps!