Primarily applying to Smith & Wesson revolvers, target trigger means an extra-wide trigger that curves gently under your finger. As the term suggests, it is intended for formal target competition.
In theory, if the trigger is wider, more of the surface of your finger will be in contact. This allegedly makes it much easier to pull. S&W used them on its top-end revolvers for decades, along with its lower, wider spur. Many shooters, who work their guns with both DA and SA trigger systems find the target trigger something of a nuisance.
I can remember one of our range masters working over an original narrow trigger to produce one with a straight ridge down the center of the finger pad. Lock the crease of the first joint of your trigger finger on the ridge and go for it! So much for theories.