What does this mean?

The "a" and "s" are what indicates applying or separating, not the positive or negative value. From my understanding, the negative sign is indicative of relative planetary positions and speed.
---

Positive applying = the faster object is behind and breaking a sweat to catch up

Negative applying = the faster object is already ahead and creating the aspect by sheer force (speed); you could consider it "pulling along", or "paving the way"

---

Positive separating = the faster object is finally ahead and has left the slower one in the dust

Negative separating = the faster object is still behind, but making tenacious headway for the next aspect; "past one hurdle, onto the next"