In a state of...

[The Medway Handyman]

...flux.

I know that you have to use it when using solder ring or end feed fittings, but what exactly is it & what does it do?

Just curious.

[R D S]

IIUC flux prevents burning on the metal surfaces (solder would not adhere well where there is oxidisation) and aids the solder in spreading out.

[John Rumm]

Keeps the metal chemically clean and prevents it oxidising. This enables to solder to "wet" the surface of the metal.

[The Natural Philosopher]

rips teh oxides off the metal you want to solder to so it flows and makes a surface (alloy? amalgam?) anyway, its the oxide ripping part that makes it work

[Slider]

It's a form of acid, cleans the copper giving a better seal with the solder

[Andy Champ]

Plumber's flux maybe, electronics people use "non-acid flux" and I've no idea what it is.

Andy

[Dave Osborne]

Rosin, usually.