Press Green Flag to begin. If you don't know what a Caesar shift cipher is, it is a code language that uses a number that shifts the places of the alphabet along. If the shift number is 3, then the normal alphabet (A, B, C, D, etc.) will become D, E, F, G, etc. For example: if the shift number is 7: Normal alphabet: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z. Cipher alphabet: H, I , J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z, A, B, C, D, E, F, G. So the message 'hello there' would become: 'olssv aolyl'.
Everything is by me, and I am sorry if the explanation is very bad (I am not very good at explaining things), but you can find a better one by googling 'Caesar shift cipher'.