You're seeing just the macro assert_sink_buffer, go back to Membrane.Testing.Assertions module for more information.
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assert_sink_buffer(pipeline, sink_name, pattern, timeout \\ 2000)

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Asserts that Membrane.Testing.Sink with name sink_name received or will receive a buffer matching pattern within the timeout period specified in milliseconds.

When the Membrane.Testing.Sink is a part of Membrane.Testing.Pipeline you can assert whether it received a buffer matching provided pattern.

{:ok, pid} = Membrane.Testing.Pipeline.start_link(%Membrane.Testing.Pipeline.Options{
  elements: [
    the_sink: %Membrane.Testing.Sink{}

You can match for exact value:

assert_sink_buffer(pid, :the_sink ,%Membrane.Buffer{payload: ^specific_payload})

You can also use pattern to extract data from the buffer:

assert_sink_buffer(pid, :sink, %Buffer{payload: <<data::16>> <> <<255>>})