Explanation

The meaning in a few plain lines

The Prophet (may Allah's peace and blessings be upon him) informs his nation about the final time of the Fajr and the ‘Asr prayers. He stated that whoever catches one Rak‘ah, meaning: he prays one Rak‘ah and raises up from the Rukū‘ in the Fajr prayer before the sun rises, then he has performed the morning prayer in its due time. Similarly, whoever catches one Rak‘ah of the ‘Asr prayer before the sun sets, then he has performed the ‘Asr prayer at its due time, because one Rak‘ah of the prayer has been performed within the prescribed time.