That issue is something called Plantar Fasciitis, which looks like two made-up words, but is actually the term for a pretty irritating foot inflammation.
Plantar Fasciitis is the term for inflammation related to the plantar fascia, the connective tissue on the sole of the foot. (runs from the heel to the ball of your foot.)
Read more about Plantar Fasciitis at the Mayo Clinic Website
Peppers, however, doesn't seem too worried about it, or at the very least, isn't willing to let on that he's worrying about it. From McClure's article:
"I'd refer to it as soreness,'' Peppers said with a smile. "General soreness.''
The old company line. General soreness.
As someone who has suffered from a minor case of this, I can assure you--it sucks. Particularly, the part where you get out of bed in the morning and almost immediately fall over because of the soreness of your foot upon your first steps.
Peppers will fight through it, though. While there's always the risk of aggravating the injury into a tear, or worse, Peppers takes pride in his durability, having started every one of his games with the Bears. Team coaches and other players rave about his performance and work ethic, along with fans admiration. In 2011, he played most of the season on a sprained MCL.
This is, however, something to keep note of. Sould he start to lose some of his explosiveness as the season goes on, or begin to favor one foot or another in his play, it could be attributed to this specific ailment.
And as we've already seen this preseason, the defensive line has enough on it's hands without losing it's best player.