Negative doubles is a convention.
It promises a responding hand (6+ points), 4 cards or longer in the unbid major (or both minors if both majors have been bid), and the inability to bid that suit directly when only one major is shown.
The double in the first example above shows 4 ♥s, 6+ points or 5+ ♥s, 6-9 points. A direct 2♥ response would mean 5+ ♥s, 10+ points in this case.
The double in the second example above shows exactly 4 ♠s, 6+ points. A direct 1♠ response would mean 5+ ♠s, 6+ points in this case.
The double in the third example above shows 4-4 in the majors. A direct major response would deny another major.