Encryption means converting content into a coded format so that unauthorized persons will be unable to see or access the original content of the file. A text file can be encrypted by applying a formula to the ASCII value of the content.
The formula or code can be of your choosing, and it can be as simple or complex as you want. For example, let's say that you have chosen to replace the current ASCII value of all letters by moving them forward 15 values. In this case, if the letter is a lowercase a that has the ASCII value of 97, then the forward shift of the ASCII values by 15 will make the encrypted letter a lowercase p, which has the ASCII value of 112 (97 + 15 = 112).