I need to know if is possible to insert a label in the atlas that is different for any pages. If I add a new label, for example, a description, it will reported on every page of the atlas with the same text, but I need to insert different notes.
Yes, you can fill any label or textbox in the composer with an expression based on attribute values.
The easiest way would be to add a fitting column in your coverage layer which is holding your notes. (Fig. 1)
In the composer, add a textbox/label, enter some static text and hit 'Insert an expression...'
The expression editor will open. Under 'Fields and values' you can double click any column to add its value (fig. 2), based on your current coverage feature, at the cursor position (fig. 3).
There are other ways of achieving more complex auto-filling tasks.
Example: For notes longer than 250 characters, you could add additional columns and concatenate them with an expression like: concat( "Note1" , ' ' , "Note2" )