Jesus took the loaves and the fish, gave thanks for them, and had them distributed to the people. They were all invited to eat as much as they wanted and the leftovers were gathered after everyone ate to their fill. Amazingly, after thousands had been fed, there was more left over than they started with, and the people recognized this as a miraculous sign.
The next day, they came seeking Jesus again. If you had experienced this miracle, what do you think you would be most content with; Jesus or more bread? Sadly, the people seemed to want bread more than they wanted Christ. They weren't thinking on a deeper, spiritual level. They were primarily consumed with their physical needs and filling their bellies.
So Jesus attempted to show them that they were seeking something of lesser importance when they should have been seeking Him. They considered consumable bread to be their primary source of life, but Jesus was trying to explain to them that He is the source of true life.