What is a hot thread?

[NogbadTheBad]

Those threads with red envelopes. The icon says its a hot thread, what defines it as hot?

[polmico]

Naked pictures of Phil Collins.

[Duncan Glenday]

15 posts and/or 150 views

[aplodon]

Those threads with red envelopes. The icon says its a hot thread, what defines it as hot?
Would it be possible to get rid of those icons (or make them much smaller) and change the display of thread titles to only go over one row and in that way have more threads on each page (alternatively, not need to scroll to see all thread titles)?