What does it mean when subs have a minus before the text?

[DVMaster]

Sometimes subtitle dialogue have a minus before the text. What does this mean? For example,

- What are you doing tomorrow?

vs.

What are you doing tomorrow?

[Ai Haibara]

Such as when it actually shows up on-screen?

I believe it's usually done when there's more than one person speaking, and whoever created the subtitles wanted to make it clear that the offset dialogue is another character's line, especially if it's not immediately clear who's speaking (and that can happen if you're paying more attention to the subs )
Or, it's used when there's other, less important (at least to the subber) dialogue in a scene, that they still wanted to translate. For example, if the main characters are holding a conversation in a school classroom, the offset dialogue might be what the 'unimportant' characters (the other students in the room) are saying.

Formatted subtitles may deal with this by using different colors/styles, moving the 'lesser' dialogue to a different part of the screen (like the top) and perhaps making the font smaller, etc.

[guns1inger]

Often it indicates a second speaker who is either off screen, or who is obscured or has their back to the camera and therefore may not be seen speaking.