وَحُكْمُهُ — “and its ruling”: “its” refers to the declinable noun from the previous lesson.
أَنْ يَخْتَلِفَ آخِرُهُ — “that its ending should vary.”
بِاخْتِلَافِ الْعَوَامِلِ — “with the variation of the governors.”
اخْتِلَافًا لَفْظِيًّا — “an outward, perceptible change.”
أَوْ تَقْدِيرِيًّا — “or an estimated change.”
Combined meaning: the governing context determines the case of a muʿrab noun, but the ending may or may not show the case vowel overtly.