Basically means you run your hand across your head and feel 0 stubble in any and all directions. Even the tiniest bit of stubble felt in any direction then technically means a "BBS" shave was not fully achieved. You also may see/hear DFS (damn fine shave) meaning stubble felt atg only, CCS (close comfortable shave) meaning you may feel some stubble atg and xtg but wtg is smooth, and SAS (socially acceptable shave) which means stubble is felt in all directions except wtg and even then some stubble/shadow maybe slightly visible. SAS is your typical quick one passer just so your head looks shaved or, if talking about face shaving, what would be acceptable as the typical "white collar shave" while not taking the time to achieve anything beyond a visually acceptable shave. Hope this helps!
More info then what I just listed can always be found at badger and blade, shave den, or sharpologist. All of which I highly suggest checking out