An "item" just means a thing. We talk about "items" in a list:
A: How many more items are left on the checklist?
B: Just three.
When someone runs a meeting, they have a list of topics that need to be discussed. You can call those topics "items of business":
So... what's the first item of business?
The list of items that need to be discussed in a meeting is called the "agenda". An agenda can be written down and formally presented to everyone in the meeting, or it can just be in the mind of the person who's running the meeting.