Allah has indeed helped you in many battlefields and (recently) on the day of Hunain: when you were proud of your great numbers (the Muslim army was 12,000 strong and the unbelievers were only 4,000), but the numbers availed you nothing. The earth, with all its vastness, seemed to close in upon you, and you turned your backs and fled.
Meaning:-
Hunayn:The Battle of Hunayn was fought between Muhammad and his followers against the Bedouin tribe of Hawazin and its subsection the Thaqif in 630 in a valley on one of the roads leading from Mecca to al-Ta’if. The battle ended in a decisive victory for the Muslims, who captured enormous spoils.
Vastness: the quality of being extremely big
flee: run away from a place or situation of danger, escape quickly from dangerous situation or place.