What type of glass fuse is this and what does the spring do?

20 amp glass cartridge type slow blow. It has a thermal mass that generates a delay before the solder melts at the left hand end. The spring pulls the mass away from the wire, making sure the two conductors separate and stay separated.

BobT has it right - my answer serves only to add information.

This class of fuse is commonly known as a "3AG" or "3AB" fuse. The Buss number would be AGC-20 (fast blow) or ABC-20 (Slow Blow).

Although they are still available with a clear glass housing, most modern versions now use an opaque ceramic housing for safety reasons (ceramic doesn't shatter or explode under extreme fault conditions).

I did a quick search at Digikey: 20A Slow-Blow 3AG fuses - 8 versions available in stock ready for immediate shipment as of this date. However, the link that I provided should be active for as long as Digikey still uses that search engine.