When a key is defined with compression of trailing spaces, trailing spaces in a key value are not stored in the file. However, information is stored so that the key can be correctly located.
For example, assume you have a prime or alternate key that is 30 characters long, and that you write a record in which only the first 10 characters of the key are used, the rest being spaces. Without compression, all 30 characters of the key are stored. With compression of trailing spaces, the key only occupies 11 bytes in the index file (10 bytes for the characters of the key and 1 byte as a count of the trailing spaces).