You're seeing just the function subscribe, go back to Membrane.Clock module for more information.
Link to this function

subscribe(clock, pid \\ self())

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subscribe(t(), subscriber :: pid()) :: :ok

Subscribes pid for receiving ratio_message_t/0 messages from the clock.

This function can be called multiple times from the same process. To unsubscribe, unsubscribe/2 should be called the same amount of times. The subscribed pid always receives one message, regardless of how many times it called subscribe/2.