what is electrical solder
Solder is basically metal wire with a low melting point, where low for our purposes means low enough to be melted with a soldering iron. For electronics, it is traditionally a mix of tin and lead. Tin has a lower melting point than Lead, so more Tin means a lower melting point. Most common lead-based solder you'll find at the gadget store will be 60Sn/40Pb (for 60% tin, 40% lead).