Multiplies the signal by mul and adds add. This UGen is very efficient (it performs various optimisation checks, for example). It is used very heavily throughout SuperCollider to perform multiply and add operations on the server; in fact it is what "really" performs the mul and add arguments found in many UGens.
See also the discussion of mul and add arguments in the UGen help file.
multiply with this value
add this value
(Note: the "trace" message is described in the helpfile for Node.)