As for the first pioneers who accepted Islam from the Muhajir (immigrants) and Ansar (supporters in Madinah) and those who follow them in good deeds, Allah is well pleased with them and they are pleased with Him. He has prepared for them gardens beneath which rivers flow, wherein they shall live forever: that is the mighty achievement.
Meaning:-
Pioneer: a person who is one of the first people to do something.
Beneath: below; in or to a lower place, position, underneath