Chelation refers to the bonding of elemental atoms that cannot normally be bonded. For example, Sodium is a metal element that can bond with many other elements, like chlorine (forming common table salt: NaCl), or hydrogen and oxygen (Sodium hydroxide). The reason for this is that the way that atoms bond with each other is similar to a jigsaw puzzle: Only the proper shape can fit into the empty space made by the other pieces. However, with atoms, it's a bit different. For each element, the atom carries a certain number of electrons in orbit around its nucleus. Sometimes these orbits are split into levels, and only a certain number of electrons can fill each orbit before moving to a higher orbit.
Sometimes, an orbit level is not filled all the way and can only be filled with electrons from another element with a smaller number of electrons in a higher level. When the atoms bond together, the electrons are traded between the two equally and create a very strong bond.
However, there are a few elements that cannot bond with many other elements, because they have orbits that are already filled, or nearly filled, equally. (Like a finished jigsaw puzzle) These elements can sometimes be poisonous to a human, and cannot be removed easily (for example, mercury.) Chelation enables a non-bonding element to bond, thereby enabling removal.