That is the valley between her breasts.

female breasts

The area between a woman's breasts is commonly referred to as the cleavage.

A cleavage is the area down the middle of an object, usually referred to the area in-between the left and right breasts of a human woman.

If she has her top off, you could drag your penis down her cleavage. You could press her breasts together as you work your penis through the cleavage. And if that arouses you to ejaculation, you could release your ejaculate wherever she likes - perhaps on the breasts or her face if she is into that.