Here's how it works:
* Will indicates future tense.
* Be is the present tense of the verb "to be," which is used in the progressive tenses.
* Singing is the present participle of "sing."
So, you would use "will be singing" to describe an action that will be in progress in the future. For example:
* "I will be singing in the choir next week."
* "They will be singing karaoke at the party."