A turntable cartridge is the mechanism that holds the stylus, which translates the grooves of a record into a signal that can become sound.
Needles, styli, cartridges, pick ups, call them what you like, every turntable has something that sits in the groove and ‘reads’ the undulations in the groove wall.
The signal produced by this process gets sent down the wires in the arm to an amplifier that equalises and boosts it to a level where it can be sent through another amplifier and into the speaker.
Do you want your records to sound better?
Then get a new cartridge.
Seriously, this is the best record player upgrade you can make. Nothing else gives you more bang for your buck, especially if you currently have a stock cartridge.
A high quality cartridge makes a world of difference.