6.11 understand that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field

If a charged particle moves into a magnetic field, it experiences a force pulling it in the direction of the field.

When a charged particle, that is parallel to the magnetic field line, entres the magnetic fiend line, it will not experience a force. This is because the charged particle is already going in the direction of the magnetic field.