The Grass referred here is the sacred grass Durva (it is called Dharbai)
The sanctity of dharba, also known as kusha (or, kusa) grass, is as old as the Indian gods.
Kusha, whose name signifies sharp in the sense of acute, is the root for the Sanskrit word for "expert," kosala. That is because the edges of the long leaves that grow in pairs along the tall stems are very sharp, so like the sword it is a symbol for discernment or "discriminating wisdom.

Even with this sacred grass Ma Sita could stop Demon king Ravana advancing towards her. Ma Sita is said to have placed the sacred grass between herself and Ravana during the period of her confinement in Lanka, and she warned him never to cross the grass. If he did, his head would burst into a thousand pieces, she told him.
Ravana fearing for his life could not cross that grass

Some say the durva grass represents the hair of Brahma; others say it is the hair of Vishnu.
Another myth explains that when the pot of Amrita was set on the sacred grass, the children of Kadru (Garuda's stepmother) were determined to get some of the elixir. Ever-watchful Garuda, to prevent their attaining immortality, quickly snatched it away. The snakes ended up licking the the leaves in hopes that some drops had fallen there, but they were so sharp that the poor serpents' tongues were sliced in two.

The unique feature of Durva grass is that it has sharp edges; Rishis used this sacred grass as potent missiles to kill demons and errant kings. The wicket king Vena was killed by Rishis using this grass as a missile.

KAkAsura episode in Srimad Ramayana, where Sri Rama, enraged at the insufferable insult and physical injury inflicted upon the Divine Mother by the misguided son of Indra, just plucked a nearby blade of grass (dharbham) and flung it at the cursed crow, uttering the BrahmAstra mantra.-

"Sa darbham samstarAt grihya BrAhmENAstrENa yOjayat

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Sa tam pradeeptam chikshEpa darbham vAyasam prati"

This simple blade of grass turned into an extremely potent weapon and hounded
the KAkAsura wherever he flew, all over the three worlds. The renowned Creator
BrahmA, the dreaded Destroyer Rudra and the Chief of Celestials Indra, the
father of the crow-none of these worthies could save the fugitive from the
pursuing BrahmAstra. After flying in vain several times around the three worlds,
the crow fell at the feet of Sri Rama, displaying utter helplessness and
remorse-"threen lOkAn samparikramya tamEvam sharanam gata:" and was saved by the KaruNA KAkuttsttha, even though he (the crow) deserved the most stringent of sentences, viz., death-"vadhArham api KAkuttsttha: kripayA paryapAlayat".

In the Trivikram Avatara too, the Lord used a Darbham to evict Shukra Acharya, who, assuming the form of an insect, obstructed the passage of water from the vessel of Mahabali, who was about to grant the boon of three feet of land sought by the wily Vamana Brahmachari

This, then, is what a blade of grass of capable of, when handled by a person of attainment