\textbf{-frame=pict\_type:} within the chosen specifier indicates the data to be displayed; in this case \textit{pict\_type}, that is, the keyframe type (I, P, B) of the frame under consideration.
-\textbf{input.mkv:} is the media to be analyzed (it can be any container and code).
+\textbf{input.mkv:} is the media to be analyzed (it can be any container and codec).
(see \url{https://ffmpeg.org/ffprobe.html} for more details)