A NOP slide or NOP sled is a slurry of NOP instructions (\x90 on x86) to make placing and executing shellcode on stack easier. It can make execution redirection easier since only a rough address is needed if we use a NOP slide (the instruction pointer could point to anything along the NOP slide, as the execution would just slide through no-ops until the shellcode). It can be used to bypass ASLR as long the NOP slide is long enough to take care of all possible random offset sizes.