ATBASH is a very common, simple cipher
It's alphabet but backwards wherein the opposite of A is Z, B is Y, C is D and so on and so forth. On the other hand, N is just N.
Let's have an example:
OB NZOW SI ILMNTWYMY
O=M B=Y
N=N Z=A O=M W=E
S=I I=S
I=S L=P M=O N=N T=G W=E Y=B M=O Y=B
So therefore, OB NZOW SI ILMNTWYMY means MY NAME IS SPONGEBOB