The understanding of AVPacket in FFmpeg?

when I read the introduction to AVPacket in the FFmpeg document, I saw this sentence:

For video, it should typically contain one compressed frame. For audio it may contain several compressed frames.
for video, it contains one compressed frame; for audio, it may contain multiple compressed frames.

AVPacket (compressed packet) is decoded into AVFrame (uncompressed packet) by the decoder.

question:
what is the relationship between AVPacket, AVFrame, Frame, and frames? How to understand the sentence in the document?

Multimedia video-processing cPP c ffmpeg
Apr.09,2021

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