Sita scorns Ravana's suggestions and says that she will never succumb to his offers.Lanka has wealth and many things to commend it,but it is goimg to be destroyed because of the sin of one man Ravana.He has abducted Her in a cowardly manner. He can never stand up to Rama's might.When Ravana is killed people will rejoice, says Sita because the world is happy to be rid of a sinner.
Sita pulls up a BLADE OF GRASS and places it before Her, as if it is barrier between Her and the demon King.And indeed the grass is a barrier.It's all about the FAITH one has in the heart for ALMIGHTY.So even a blade of grass is a potent weapon where the Lord is concerned.The Lord emanated from a pillar to save Prahalada. Just as He is present in pillar,so is He present in a Blade of grass, and can emerge out of anything and anywhere to save his devotees.
Faith is a big thing and your faith can be so powerful that it can move mountains.So the grass that Sita places before her, is not a mere blade of grass, but a lot more.It's the reflection of Her Faith and Devotion.