Subservient means "compliant," "obedient," "submissive," or having the qualities of a servant. Something that's subservient has been made useful, or put into the service of, something else.
Our word subservient is unusual because it actually comes from the Latin word of exactly the same spelling and almost the identical meaning — "subjecting to" or "complying with." Usually words change a little in spelling or meaning when they are adopted from another language, but this one did not. Maybe the word subservient seemed so subservient that it could comply with the rules of any language!