What's the difference between [Save] & [Apply] button ?

I love grafana very much, it is an extremely elegant sofeware.

After creating my monitor panel, i’d like to edit alert rule, but the alert not make effect while the metric reaching the thresholds. After a lot of google searching, i solve it by Click [save] button, before it, i just click [apply] button after edit. So, i want to know the difference between [Save] & [Apply] button.

Anyone can explain it for me ?