To encode VBR mp3 audio with FFmpeg, using libmp3lame library, we would specify a command like this:

ffmpeg -i input.wav -aq 2 out.mp3

Option "-aq" needs a parameter, that tells FFmpeg something about expected "audio quality" (of course, this parameter has a different meaning for each audio encoder used, and we are now describing the libmp3lame encoder).

The table for LAME option "-V <num>" is shown here (and that option is mapped to FFmpeg's "-aq" option):  http://wiki.hydrogenaudio.org/index.php?title=LAME#VBR_.28variable_bitrate.29_settings

Switch        Kbit/s        Bitrate range kbit/s
-b 320        320           320 CBR
-V 0          245           220...260
-V 1          225           190...250
-V 2          190           170...210
-V 3          175           150...195
-V 4          165           140...185
-V 5          130           120...150
-V 6          115           100...130
-V 7          100            80...120
-V 8           85            70...105
-V 9           65            45...85

So, in our example above, we selected "-aq 2", meaning we used LAME's option "-V 2", which gives a VBR mp3 audio in the range of 170-210 kbit/s.

If you want constant bit rate (CBR) mp3 audio, instead of using "-aq" option, you need to use "-ab" option to set the preferred "audio bit rate" in bps, like "-ab 256k" if you want 256 kbit/s audio.