To pilfer is to steal something, typically of small value. Minor thefts, like taking a roll of toilet paper out of a public bathroom or napkins from the Early Bird Buffet are what your grandfather, for example, might pilfer.
The verb pilfer comes from the Old French noun pelfre, meaning “booty,” or “spoils.” Now pilfer is used when talking about the act of stealing loot: you may find that you have to really restrain yourself from the desire to pilfer your friend’s new fur-lined gloves, even though you're pretty sure she pilfered your headphones. Robin Hood was able to pilfer from the rich to give to the poor. How romantic!