What is a standalone movie? Why do they appear to be merging two words together?
It probably means that they’re not planning any sequels, and may not tie it into the greater Marvel cinematic universe.
In the Marvel film universe, movies typically reference events from other movies, or build upon events from earlier movies. My guess is they are not seeking to tie the Black Widow movie into the evolving story line.
More people are likely to see Black Widow than Black Panther.