A chase figure is a toy that is released in limited amounts relative to the rest of an assortment, often something like "one chase figure for every two cases of regular product" or similar. The name comes from the assumption that collectors, in their need to "collect them all" will put in more effort than usual to "chase" down said toys. Chase figures are relatively rare in the Transformers toyline, although they are common in other series.
Only a handful of Transformers toys and products could really be considered "chase" figures, mostly some of Takara's PVC and "World's Smallest Transformers" series, and some figures from the Micron Booster series.
See also: Lucky Draw figure