Clear caching notifications

On Hummingbird, I have Browser Caching activated.

I do NOT have Page Caching activated.

I have used Hummingbird for about 2 years and never received a notice about caching. Recently, I have been receiving this notice regularly on all 5 of my sites:

“We've noticed you've made changes to your website so you might want to clear Hummingbird's cache to avoid any issues.”

Why?

What does it mean?

Thanks in advance ...