To become less crowded or to become empty — used for seats, shops, roads, etc.
昼休みになると駅のホームが少し空く。
The station platform becomes a bit less crowded at lunchtime.
夜遅くなるとこの通りはほとんど空く。
This street is almost empty late at night.
コンサートのチケットはまだ若干空いている席があります。
There are still a few vacant seats for the concert.
To become hungry (lit. "the stomach becomes empty"). Often used as "onaka ga suita."
授業の後でいつもお腹が空く。
I'm always hungry after class.
まだ昼前なのにもうお腹が空いた。
It's still before noon, but I'm already hungry.
長い会議でみんなお腹が空いてきた。
After the long meeting everyone started getting hungry.