Sequestering agents are also known as chelating agents. They form two or more separate bonds between a polydentate (multiple-bonded) ligand (usually an organic compound) and a single central atom. The central atom is thus "surrounded" by the sequestering agent and deactivated.
They are used to bind calcium to soften water and to bind ions that lead to discolouration, etc.
EDTA, citric acid and phosphonates are well known sequestering agents.