Short Answer: Lennie wants to eat his beans with ketchup.
In the opening scene of the novella, George and Lennie walk down a worn path to the banks of the Salinas River, where they rest before heading to the new ranch the next day. When Lennie asks George why they don't head to the ranch that night to receive supper, George explains that he saw thrashing machines on the walk down, which means that they will be in for an evening of arduous labor. George then instructs Lennie to gather some dead willow sticks in order to start a small camping fire and tells him that they will have the three cans of beans from his bindle for supper. Lennie responds by telling George that he likes his beans with ketchup. George then explains that they have no ketchup and tells Lennie to fetch the dead willow sticks for the fire.
In the novel Of Mice and Men, Lennie and George settle down close to the Salinas River before they head to the new ranch they are to work at.
George tells Lennie that they are going to have a supper of beans. Immediately, Lennie states
"I like beans with ketchup."
George tells Lennie that they do not have any ketchup. Because of the fight over the mouse, and the lack of ketchup, Lennie becomes somewhat distraught. Lennie threatens George with running off and letting George go on without him.
George, finally realizing Lennie's despair, tells Lennie that he wants him to stay. Lennie goes on to tell George that if they had ketchup, he would not eat any of it. Lennie tells George that he would "not touch any of it" and leave it for George to cover his own beans with.