What does "delete oldest backup" mean?

enahs wrote:
My chief interest in Time Machine is to backup the User file, especially the iPhoto library. I've also backed up the applications.

If you have room, please consider letting Time Machine back up your entire system. That's what it was designed for, and protects you best that way.

I guess what I am asking is whether I have anything to worry about as the disk gets full and a deletion takes place.

In a word, no.

Time Machine backups are very different from traditional backups. After the first full backup, yes, it only backs-up what's new or changed since the last one. But it makes "hard links" (sort of like aliases) to the backups of the things that didn't change. That means that every backup is, in effect, a full backup, and is how TM can show you a "snapshot" of your entire system the way it was at the time of any backup.

When it deletes a backup, it deletes that set of "hard links." The only time an actual backup copy is deleted is when there are no more links to it (meaning, it was not on your system at the time of any remaining backup).

So TM will always have backups of the current contents of your Mac, plus previous versions of things you've changed or deleted for as long as there's room.

You might want to review these:

Time Machine Tutorial
Time Machine 101
How to back up and restore your files
Time Machine Features
Apple - Support - Mac OSX v10.5 Leopard Time Machine

and perhaps browse the Time Machine - Frequently Asked Questions *User Tip* at the top of this forum.