"We use the future continuous to talk about [an action happening at the some point in the future]".
In that example, the bracketed sequence is a noun phrase in which "action" is the head (main) word that is being modified by the underlined non-finite clause. The verb "is" is not required with non-finite clauses modify nouns. You could replace the non-finite clause with a finite relative clause with no change in meaning:
"We use the future continuous to talk about [an action that is happening at some point in the future]".