Remember when Jesus went alone in the mountain while the disciples went boating? The multitudes were now looking for Jesus. They were pretty serious in figuring out where Jesus was because they considered boat location, which led them to go boating towards Capernaum in search for Jesus. Eventually, they found Jesus and asked him on when Jesus came to the current location at that time.
However, Jesus pointed out on how the peeps were seeking Jesus curiously likely because they were among the 5000 that were fed a few verses ago. It seems as if the peeps have more focus on Jesus for meeting personal wants (kingship and food) rather than learning the character of Jesus. Thus, Jesus told them to focus on the "food" from the Son of Man rather than the ordinary food.
He wanted the peeps to focus on Jesus's food for two reasons. First, the food of the Son of Man lasts, while the other food doesn't last (6:27). It's common for us to focus on material satisfaction rather than Jesus and his "food" (which, as we'll see, isn't literal food). Sure, material satisfaction is nice, but it only lasts so long. And when we over-rely on material satisfaction, it would be hard for us to feel sustained when we don't have such satisfaction. That's why Jesus wants to share his "food" with us -- the "food" lasts. Additionally, the Father "has set His seal on [the Son]", so the food that the Son gives is something that the Father approves of (6:27). In the next few verses, Jesus would give further explanations on the "food" and how to obtain it.