Button mashing often refers to the act of quickly pressing various controller inputs in rapid succession. While carrying some negative connotations in fighting games, Smash included, button mashing does have some specific uses. For example, rapidly pressing buttons allows characters to escape situations like being stunned, grabbed, or frozen more quickly.

BSD: https://www.youtube.com/watch?v=pwfXtDiQA24&t=234s
Wiki: http://www.ssbwiki.com/Button_mashing