WP:SYNTH is one of wikipedia's policies. It essentially says not to combine sources to form a conclusion. You need to accurately summarize what sources say. So drawing conclusions, extrapolating, writing down unstated inferences, are all SYNTH. Short version: "Don't make shit up using sources"
This policy can never be used in AFD. That's right, it can never be a reason to delete an article. Why not? Because the sources are there. If there are no sources, it's Original Research (a different policy), not SYNTH. If there are sources, then the article has sources, but doesn't summarize them correctly. The answer is to FIX IT, not delete it. Since the article has sources, it shouldn't be deleted. If it should be deleted despite having sources, that's a sourcing problem, and again, is a different policy (ie doesn't meet GNG, not enough sources, poor sources), NOT SYNTH.
Conclusion: DO NOT USE SYNTH IN AFD. EVER.