Why am I getting a '+' sign when chosing a character format??

By "format", do you mean a Character or Paragraph Style?

The + at the end of a Character/Paragraph style name in the panels indicates there are overrides - things which are different than the style. Therefore Illustrator is ensuring it's not changing anything it shouldn't - and then adding the + as a visual indication that not everything in the text matches the style settings.

For example.. if you have a paragraph set as "Style 1" and within that paragraph, you've changed a couple words to make them bold or italic. Since bold and italic is not part of the paragraph style, they are overrides, and the + will show.

You can clear overrides in the Character/Paragraph styles by Option/Alt clicking the style in the appropriate panel. Doing this removes all overrides and resets everything to match the style settings. Without the Option/Alt key, clicking a style name reset everything to the style settings except anything which is already designated as an override.