When typing on a Paragraph Text layer, it's possible to add more text than can fit inside the bounding box. If you do this (or if you resize the bounding box so it's too small for the text you added earlier), After Effects will hide some of the text. The overflow symbol indicates there's hidden text. To reveal hidden text, either decrease the font size (so more of the text fits in its bounding area) or increase the size of the bounding area (using the Type tool). When the overflow symbol vanishes, there's no more hidden text.