Chelation and its relation to transition metal ligands
I'm giving the answer thinking that you are familiar with ligands and Denticity. 

Polydentate ligands form two or more coordinate bond with the central metal atom. For eg:EDTA can form up-to 6 coordinate bond with the central atom. In these situations a single ligand encloses the metal ion like enclosing it in a cage. This is called chelation. Co-ordination compounds with chelated metal ions are more stable then those in which the metal ions are surrounded by many monodentate ligands.If you want more information read the Wikipedia article on this http://en.wikipedia.org/wiki/Chelation.