What is an eip register?
There are a ton of questions on here that make reference to the eip:
How can I partially overwrite the EIP in order to bypass ASLR?
Unable to overwrite EIP register Do I always have to overwrite EIP to get to write on the stack in a buffer overflow? Etc. What is the EIP? How is it used, both as an exploit target and in benign code?
EIP is a register in x86 architectures (32bit). It holds the "Extended Instruction Pointer" for the stack. In other words, it tells the computer where to go next to execute the next command and controls the flow of a program. Research Assembly language to get a better understanding of how EIP register works. Skull Security has a good primer.