A turntable plays various size records. It outputs audio via RCA cables (the red and white audio cables). These cables carry an analogue signal that must be converted to a digital signal when recording to a computer. A USB adaptor connects to a USB port and acts like the computer's sound card. Many USB adaptors can accept RCA inputs that the adaptor translates into a digital signal for use with the computer.
Connect the red and white connectors on the RCA cable to the RCA jacks on the back of the turntable.
Connect the other end of the RCA cables into a RCA input on the USB adaptor.
Connect a USB cable from the USB adaptor to an open USB port on the computer.
There are many turntables available now that have direct USB connectivity.