my mom says: there are no stupid questions.... only stupid answers

summing is simply mixing signals together to eg. stereo

it can be done in the protools mixer .... an external mixer .... or a dedicated external summing mixer that does nothing more than adding all the signals together.....

alot of talk has been about the dedicated summing mixers as they can make a protools mix sound better than summing digitally (protools)....

and remember.... we're all here to learn

Summing can also be mixing two or more channels or tracks to one or more channels, tracks or sub mix bus's ..

Mixers all have summing amplifiers .. that combine the signals of many channels into one. An aux is a summing bus..as is the 2 mix (stereo master) or a sub. Any time you are adding two or more signals together, you are summing ... just like in arithmetic..

summing can have negitive effects as well. usualy assosiated with budget equipment, if you say use the same compressor with the same settings over and over and over on every track and then on the mixed master that pleasant litle 2Khz hump in the freaquency resonce will really start to stand out.