“Net approval rating” means the percentage of Trump “approvers” minus the percentage of “disapprovers.” To state the obvious: if a given state has more approvers than disapprovers, the number is positive. But if the disapprovers outnumber the approvers, then the “net approval rate” is a negative number.
The numbers on the right show Trump’s “net approval rate” as of January 2017. The numbers on the left show “net approval rate” as of now.
For example, in January, 2017, in the state of New Hampshire, Trump’s “net approval rating” was plus one. Now it’s minus 17.
You will observe that in all these “battleground” states the arrows are pretty long and they all point left.