You are programming a buffer overflow exploit and you want to create a NOP sled...

A NOP (No Operation) instruction tells the CPU to do nothing for one instruction cycle. In exploit development:

A NOP sled is a long sequence of NOPs (0x90) that increases the chance of the instruction pointer landing before the shellcode.

0x90 is the x86 opcode for the NOP instruction.

From CEH v13 Courseware:

Module 6: Malware Threats → Shellcode and Buffer Overflow