It's the average round a 1000 rated player will throw on a given set of tees.
The SSA (scratch scoring average) is calculated by comparing how the players did versus their actual PDGA rating. Using these two values, we can deduce what score a 1000 rated player would have, and that becomes the SSA. Once you have an SSA (which is basically the score for a 1000 rated round), all players (including those who don't have PDGA ratings) can have their round rating calculated.
For example my home course white tees have an SSA of 48-51, depending on conditions. If the SSA that day is 49 and I shoot a 49, I just shot a 1000 rated round. For every stroke off of SSA I am, approximately (depends on course / conditions) 10pts are deducted.