In simple words, a Microsoft Signature Edition PC is a clean PC without any bloatware/junkware/trialware etc.
Normally when we buy any new pc, we usually get a lot of softwares pre installed including Anti Virus applications, DVD writing applications, Music players etc etc. Manufacturers may include these as a result of their tie ups with the companies or they think their users may need them.
But the fact is many users do not need need these and most functionality exists by default in windows. This unnecessary software is a burden on the system resources and adds to the startup/shutdown times apart from also slowing down the pc for general use. Some pcs come with so many trial, bloat softwares that the menus/metro icons feel cluttered right out the box even before we install anything. The performance drop may not be noticeable all the time, but it exists.
Power users prefer to do a clean install on their new pc. Some users just uninstall what they do not want. Some users ignore these.
So, Microsoft has come out with the Signature Edition concept(in 2012?). The signature edition pcs come with a clean install of Windows. You will not get any unnecessary trial wares or other bloat ware. As a result it should be faster..
In some cases Manufacturers may bundle their own software with their Signature Edition PCs. I've seen Toshiba bundling TruNote, TruCapture, TruVoice(I hope I remember the names correctly) with their Encore 2 Write tablets. These are not trial ware, these are full applications that Toshiba developed for use on their Encore write tablets and so they bundle them with the tablets.
Head to Microsoft store if you want a Signature Edition PC. You may not find all the models, but if there is something you like, it is worth buying from here.