The lac repressor is an enzyme which stops the metabolism of lactose in bacteria as long as glucose is available. When glucose is scarce and lactose is plentiful the lac repressor becomes inactive to permit the conversion of lactose to glucose.
Human DNA encodes recipes for making various types of RNA which in turn make various types of proteins. But human cells need a way to select the appropriate recipes for the current ambient conditions. For this humans need regulator molecules that act like factory foremen. They monitor the environment and use the information to signal to the cell. An enhancer tells the cell to crank up production of a particular substance. A repressor tells the cell to reduce production.
The first step in production is for RNA polymerase to make RNA from a stretch of DNA. DNA can be thought of as a highway, with entrances and exits. The enzyme RNA polymerase is like a car. For a specific protein sequence to be made, the car must drive from a specific entrance and get off at the next exit. The repressor in its active form is like a red light at the entrance. The lac repressor works with the cooperation of another regulatory protein that responds to glucose and activates the lactose path only when glucose is low. It is like a green light at the entrace.
The reason the lac repressor can do this is because it can take two forms. In one of these two forms it can lock onto the DNA and block RNA polymerase. Electrostatic forces hold it in place. When lactose is present the lac repressor hooks up to lactose and changes shape and detaches from the DNA. The technical term for this shape-shifting is "allosteric signal transduction".