What is "activity" in solder flux?
Flux &quot;eats&quot; oxidation.  You can't make a soldered connection to oxidized metal - it has to be bare, clean metal.
Flux removes the oxidized layer, and prevents the bare metal from oxidizing for a little while.  The melted flux layer keeps air (and therefore oxygen) away from the cleaned surface.
The &quot;activity&quot; says how aggressive the flux is.  Higher activated = removes oxidation faster.
The downside is that a more aggressive flux also &quot;eats&quot; more metal from the surface and from the soldering iron tip.
At an extreme, acid flux as is typically used in plumbing will eat the traces right off the board.