Chia seeds (grain product, surprisingly) are seeds from the plant Salvia hispanica that are ground and used for supplemental purposes to supply dietary fiber and fatty acids. The fiber component is mostly insoluble and absorbs a large amount of water (similar to psyllium husk, comparisons between the two not conducted) while the fatty acid component tends to be mostly omega-3 fatty acids (60% overall and as alpha-linoleic acid) and some omega-6 fatty acids (20% overall and as linolenic acid). There are some phenolics in chia as well, with Myricetin being the most plentiful one.
For all intents and purposes, currently chia supplementation is only really supported for the fiber aspect and even this is not overly well supported. Dietary inclusion of chia (assuming calories are kept the same) has mixed evidence for some health parameters and null evidence for other parameters and currently no human evidence for weight loss. A reduction in appetite has been noted once (common to dietary fiber) but this does not appear to reduce weight over longer trials where diet is not controlled.