This has been popping up
 

Don't know what it means.Attachment 1544

Re: This has been popping up
 

That is just a reminder that if you want to use the feature for removing the silence from the start or end of a file that you haven't done it.
If you look in the Tools menu you will see an option for Remove Silence. Opening the dialog the bottom part has two options, normal play and remove silence, the later part will only notice any difference if you have previously used the start option at the top which processes all the timings for removing the silence.
Be aware this can take considerable time although you don't have to complete it in one go.

Re: This has been popping up
 

If you don't want to this to pop up anymore, just tick the box "Don't ask me again." (I did)

Also, as Roy pointed out, if you've never removed silence, you will have a lot of songs to do, but you can stop at any time and resume later.

It will then start over where you left off until you get them all done...