Labour Value is the time value of the labour that goes into the creation of products and the provision of services. If it takes you one hour to bake a loaf of bread, then the Labour Value of that bread is one hour of your time.
Adam Smith, "father of Capitalism," discusses the labour theory of value in his seminal book, The Wealth of Nations
A few quotes from the book
Money contains "The value of a certain quantity of labour which we exchange for what is supposed at the time to contain the value of an equal quantity..." Adam Smith, The Wealth of Nations
"The value of any commodity, therefore, to the person who possesses it, and who means not to use or consume it himself, but to exchange it for other commodities, is equal to the quantity of labor which it enables him to purchase or command. Labor, therefore, is the real measure of the exchangeable value of all commodities." Adam Smith - The Wealth of Nations
"Labor alone, therefore, never varying in its own value, is alone the ultimate and real standard by which the value of all commodities can at all times and places be estimated and compared. It is their real price; money is their nominal price." Adam Smith - the Wealth of Nations
- Smith, Adam. The Wealth of Nations - An Inquiry into the Nature and Causes of the Wealth of Nations. Bantam Classics, 2003.