That is a 6.35 mm (1/4-inch) TRS audio jack plug
What it is used for
It’s an audio connector commonly used for:
Headphones
Musical instruments (guitar, keyboard)
Amplifiers
Mixers and audio interfaces
Key details
Size: 6.35 mm (also called 1⁄4-inch)
Type: TRS (Tip–Ring–Sleeve)
Tip = Left audio
Ring = Right audio
Sleeve = Ground
Gold-plated: Better sound quality and corrosion resistance
Blue casing: Usually a repair or replacement plug where you connect or solder audio wires inside