picture->pkt_dts = p->avpkt->dts;

This line of code sets the `pkt_dts` field of a `picture` struct to the `dts` field of the `avpkt` struct of another struct `p`.

`pkt_dts` represents the Decoding Time Stamp of a video frame, which is the timestamp indicating when the frame should be decoded and displayed. `avpkt` is an `AVPacket` struct in the FFmpeg library, which contains compressed data of a video or audio frame. `dts` stands for Decoding Time Stamp and is a field in the `AVPacket` struct that holds the decoding timestamp of the packet.

Therefore, this line of code is used to assign the decoding timestamp of a video frame to the `pkt_dts` field of a `picture` struct, which can be used in further processing of the video frames.