pinentry-qt is a program that allows for secure entry of PINs or pass phrases. That means it tries to take care that the entered information is not swapped to disk or temporarily stored anywhere. This functionality is particularly useful for entering pass phrases when using encryption software such as GnuPG or e-mail clients using the same. It uses an open protocol and is therefore not tied to particular software.
Pinentry-qt implements a PIN entry dialog using the Qt tool kit, so it is especially suitable for users of KDE. If the X Window System is not active then an alternative text-mode dialog will be used.
Pinentry-qt is typically used internally by gpg-agent. Users don’t normally have a reason to call it directly.