A spring wherefrom the servants of Allah will drink, causing it to gush forth abundantly.
meaning, this (drink) that will be mixed for these righteous people will be taken from Kafur, which is a spring that Allah's close servants will drink directly from, without it being mixed with anything, and they will drink to their fill from it. The word Yashrabu (to drink) includes the meaning of Yarwa (to quench one's thirst).
Allah then says,
يُفَجِّرُونَهَا تَفْجِيرًا
causing it to gush forth abundantly (Tafjir).
meaning, they will have control of it however and wherever they wish. They will have access to it from their castles, their homes, their sitting rooms and their residences.
At-Tafjir means to cause to gush forth or flow out. This is as Allah says,
وَقَالُواْ لَن نُّوْمِنَ لَكَ حَتَّى تَفْجُرَ لَنَا مِنَ الاٌّرْضِ يَنْبُوعًا
And they say;"We shall not believe in you until you cause a spring to gush forth from the earth for us. (17;90)
And Allah says,
وَفَجَّرْنَا خِلَـلَهُمَا نَهَراً
We caused a river to gush forth in the midst of them. (18;33)
Mujahid said,
يُفَجِّرُونَهَا تَفْجِيرًا
causing it to gush forth abundantly.
"This means that they will divert it to wherever they wish."
Ikrimah and Qatadah both made similar statements.
Ath-Thawri said,
"They will cause it to flow wherever they wish."
The Deeds of these Righteous People
Allah says,
يُوفُونَ بِالنَّذْرِ وَيَخَافُونَ يَوْمًا كَانَ شَرُّهُ مُسْتَطِيرًا