Backdrop blur softens the scene behind a partly see-through panel. It is like looking through frosted glass: shapes remain visible, but fine details are blurred.
A floating toolbar might use this effect so the page behind it is still present without competing strongly with the controls. The panel needs some transparency for the softened background to show through.
Check that text and buttons remain easy to read against whatever appears behind the panel. Blur is decorative; use a solid background if the effect makes the content unclear.