This cartoon is based on the idiom collecting dust.
If something is collecting (or gathering) dust, it isn't being used any more. Dust is the fine dirt that builds up on surfaces that have not recently been cleaned. Objects become dusty if they are not used for a long time.
Examples:
My brother bought an expensive tennis racquet last year but he never uses it. It's just collecting dust in the corner of his room.
My violin has been collecting/gathering dust since I broke my finger.
I found my father's old books collecting/gathering dust in my grandmother's basement.
A vacuum cleaner is a machine cleans floors and other surfaces by sucking up dust and dirt.
