In short
A junction marked with criss-cross yellow road markings; do not enter unless your exit is clear.
A box junction is a road junction marked with criss-cross yellow lines. Drivers must not enter the box unless their exit is clear, with one exception: when turning right and held up only by oncoming traffic, you may wait in the box.
Entering a box junction with a blocked exit is a serious fault on the UK practical test and a finable offence outside the test.