I've notices that when the pointer hovers over the down-vote button there is a popup that says: "This question doesn't show any research effort; it is unclear,or not useful." Is that what down-voting a post is meant to indicate? Just curious.
This question comes up on many of the StackExchange sites; see for example here for example, which in turns references another site. The wording is fixed across sites, and is intended to provide a helpful reminder to someone thinking about up- or down-voting.
There has also been discussion in the past about changing the tooltip, particularly for Meta sites where up- and down-voting often carries a connotation of agreeing or disagreeing with a proposal; see here for example (that's a duplicate but I think has more useful discussion than the main question it links to).
So directly answering: an upvote on the main site should indicate that the question has made some effort (not necessarily research, depending on the site), or has value in some clearly definable way. A downvote, contrariwise, indicates that insufficient effort has been made in producing the question. On Meta, votes may in fact be indicating approval or disapproval instead, especially if the feature-request tag is present.