What is tinned copper wire?

STRANDED BARE COPPER WIRE & TINNED COPPER WIRE

UL3173 Wire: 14/1C 41 Strands Tinned Copper SIS 90C Blue

Ancor Red 18 AWG Tinned Copper Wire - 100'

14 Ga Pre-Tinned Copper Wire - 4 Oz

Tinned Copper VS Bare Copper - Pacer Group

Pre-tinned wire has many uses in stained glass, jewelry and soldering projects Sold in a 4 ounce spool

20 Gauge Pre-Tinned Copper Wire - 4 oz

Artistic Wire, 20 Gauge Tarnish Resistant Tinned

1 pound (16 oz) Tinned Copper Wire (silver color) 16 Gauge - 126 feet

1 pound (16 oz) Tinned Copper Wire (silver color) 18 Gauge - 199 feet

Bare vs. Tinned Copper Wires

Tinned Copper Wire Manufacturer in India

Item # 1/0-01BC1901, Tinned Copper - Stranded On American Wire Group

Orvac Qualitech UL1007 16 AWG Gauge Stranded Hook-Up Wire, 300V

Why does tin coated copper wire produce better solder joints than