Gardens of dense shrubbery: They are gardens which are filled with huge dense trees.
The wordغُلْبًا (of dense shrubbery) is used as a metaphor to show hugeness. The original meaning of the Arabic word is the hypertrophy of the neck.
From here, we see that the elements of life are not only limited to food and drink, but there is also a need for forests and wood, as wood is used for many agricultural activities that human life would be unable to go on without.
أَبًّا (Grass): That livestock graze on.
There is a story concerning this word connected to Sayyidina Abu Bakr As-Siddiq and Sayyidina Umar Ibn al-Khattab (May Allah be pleased with them) who are scholars in understanding the Book of Allah (swt). This story as narrated by Ibrahim At-Tamimi (May Allah be pleased with him):
When Sayyidina Abu Bakr As-Siddiq was questioned regarding the meaning of the word أَبًّا (Abban) (Grass), he answered: “Which earth will carry me and which sky will shade me, if I say something about the Qur’an according to my own opinion?”
Here, it is as if he did not know so he stops at this point and did not want to go into discussing its meaning. This is a lesson on being trustworthy with regards to knowledge. Sayyidina Umar Ibn al-Khattab, as well, did not answer when asked about it.
This trustworthiness with regards to knowledge has played a very important part in transmission from the Prophet (saws). These great men, such as Sayyidina Abu Bakr, Umar, Uthman, Ali and the rest of the Companions (May Allah be pleased with them), were loyal trustees over each letter and every word found in the Book of Allah (swt) and the Sunnah of His Prophet (saws).
Whereas, nowadays you may find people who give answers whether they know or not and they say what they do not have knowledge about. We have to take into consideration the Prophet’s saying:
“Whosoever talks about the Qur’an using his own opinions should prepare his seat in the hellfire”.
 Kinz al-Ummal, Chapter 2, surah Abasa, Hadith No. 4688
 Sunan at-Tirmizi: The Book of Tafseer, Chapter: The one who interprets the Qur’an with his own opinion, Hadith no. 2951