Chemically combining with a metallic ion into a "coordination complex". Common chemicals used for chelation are EDTA (Ethylendiaminetetraacidic acid) and DTPA (Diethylenetetraminepentaacidic acid).
this can be used in a general way "Mary has been chelating her son." or it can be used to refer to just the time period when one is actively doing chelation (the "on" part of the cycle), e.g. "I am chelating today".