ffmpeg -f lavfi -i "anullsrc=sample_rate=48000:nb_samples=1024:channel_layout=mono" -c:a aac -y -t 60 silence.m4a
-f lavfi indicates that the next in put (-i) will be a filter source, not a network or file source.
-i “…” is the input filter and its parameters. The first anullsrc is the audio source name, and everything that follows is in the syntax parameter=value separated by colons, and the whole filter string should be quoted.
The audio source parameters are fairly straight-forward. sample_rate=48000 sets the audio sampling rate to 48kHz, channel_layout=mono creates an audio source with one channel. This could also be stereo. nb_samples=1024 means the filter will deliver 1024 samples at a time to the encoding pipeline.
-c:a aac encodes the input audio as AAC
-y means if silence.m4a already exists, overwrite it.
-t 60 means to limit the output duration to 60 seconds
The final parameter silence.m4a is the output file.