A Caesar Cipher is a way of encoding text by rotating letters throughout the alphabet.
Typical stories that involve Caesar Ciphers will give the Detective encoded text, and some clue as to how far to shift each letter. Here's an example:
Please forgive this mysterious way of contacting you, but this is a matter of the utmost urgency and secrecy. You must locate me before something disastrous happens! To find out where you must go, there is little you must know. I can give you this rhyming clue, letters you must shift by ?
The Detective would then be given the nonsense letters RCTM.
To decode this they'd have to recgonize it as a Caesar Cipher, and then realize that the only number that rhymes with clue is two, and then shift the letters backwards by two. The R becomes P, the C becomes A, the T becomes R, and the M becomes K. So the decoded word is PARK.