What does "the best thing since sliced bread" mean — and why is it funny?

informal, humorous

Meaning

A jokey way of praising something as a wonderful new invention or idea.

Where it comes from

Sliced bread was sold by machine from the 1920s and advertised as a great leap forward; the phrase grew up gently mocking that grand claim.

Why it is funny

The humor is the modest height of the bar. Every new marvel gets measured against pre-cut bread, which quietly suggests human progress peaked at the bakery.

Used in a sentence

"He just discovered slow cookers and thinks they're the best thing since sliced bread."