A Streak of Lean is a piece of pork fat with a very small amount of meat in it.
You use it as a cooking fat.
Used in cooking in the southern states of America.
You put pieces of Streak of Lean in a frying fan, and cook them to render the fat out, about 20 minutes over low heat.
When the fat is out, you raise the heat to medium, cook the pieces of meat, then remove them and either eat them or use them in a recipe.
You then proceed to cook what you are cooking using the fat in the pan.