Waterproof Solder Seal Heatshrink Butt Connector specifically a KT Cables Waterproof Solder Joiner, typically used for joining or repairing wires.
Features: 

Function: Connects and seals wires without crimping.
Material: Polyolefin tubing with hot melt adhesive and a solder ring inside.
Waterproof: Provides a secure, water-resistant seal when heated.
Application: Commonly used in automotive and marine applications.
Process: Heat is applied to melt the solder and shrink the tubing, creating a sealed connection.
Inspection: Semi-transparent tubing allows for visual inspection of the connection.
Durability: Protects against abrasion, vibration, chemicals, and saltwater.

Applications:

Electrical Termination:
Solder sleeves provide electrical termination in various applications, such as terminating wires to component terminals and splicing wires. 
Wire Connections:
They are designed for wire connections in environments with extreme temperatures and resistance to fluids. They are suitable for marine, automotive, and aviation applications. 
Wire Splicing:
Solder sleeves offer a safe and economical solution for joining or splicing two wires together. They can also be used for buried joints. 
Waterproofing:
These connectors ensure a strong, waterproof connection with strain relief. They are ideal for use in marine and other harsh environments. 
Automotive Repairs:
Solder sleeves are useful for repairing wiring in vehicles, especially in tight spaces where crimping is difficult. 
General Use:
They can be used for various applications requiring a secure and insulated connection, such as in trucks, buses, trailers, and other vehicles. 

The different colors (red, blue, and yellow) of the solder sleeves can be used for easy identification based on wire size. The transparent tubing allows for precise inspection during installation.