If you are peckish, you are a little bit hungry — not starving, just ready for a snack. This is British English, and it is friendly and informal. You might say it mid-morning, before a meal, or at tea time. American speakers usually understand it, but they do not use it much; they are more likely to say “a little hungry.” Though actually, it is a soft, polite hunger: people say it when they want a biscuit, not a feast.