can someone translate this for me

[ghoalex]

Looking through some cipher code and I was hoping i can get a better understanding of this equation;

if (ch >= 'A' && ch <= 'Z') //If the character is in the alphabet...
            {
                ch = 'A' + (((ch - 'A') + shift + 26) % 26);
            }
            if (ch >= 'a' && ch <= 'z') //If the character is in the alphabet...
            {
                ch = 'a' + (((ch - 'a') + shift + 26) % 26);
            }

mainly the whole ch = a + ch - a
if ch is f then would f + a = g?

[48T7M4Gy]

This is the Caesar encoding of uppercase and then lowercase characters of the alphabet with a shift value of 'shift'.