Thats sticky back insulation, 3mm thick 50mm wide.
Applying this to the insulation is the key to ensuring the socket is air tight. If you do a good job grease isnt needed in the socket i find :up:
But always recommend ppl still use grease just in case, or atleast on the back of the cpu so if there is moisture you dont a get rusty cpu.
Just before that i use cork tape or seal string to fill that small gap. I have also stuck it on the retaining clip as well in the past.