"DD" stands for Dolby Digital . While your text says "1.5," it is likely a typo for 5.1 , which refers to surround sound (five speakers and one subwoofer).
This is the "tag" or handle of the group or person who uploaded the file. In this case, it likely refers to a specific Telegram channel or file-sharing community. [ @WOMBACKUP ] Iron Man 2 2010 Hindi DD 1.5 480p
This specific text string looks like a standard used by online movie uploaders and "encoders." If you are looking to understand what each part of this label means, "DD" stands for Dolby Digital