Pinfu is a standard yaku. This yaku is defined by having 0-fu aside from the base 20. Though, this yaku may receive the bonus 10-fu acquired from a closed ron. This yaku is closed only.
In English, pinfu is commonly referred as "all sequences". However, the yaku is not as simple as that. Other than having an open hand, it is possible for a hand to be composed of all sequences and still not be classed as a pinfu.
To understand pinfu, players must understand the nature of fu. Fu are a set of points associated with hand composition. Pinfu lacks any of these.