The arrows are pointing to the box at the top of this page. This box appears on every page on the site. By clicking the mod name of your choice, you will find out information on that particular mod. Of course, because these links are on every page, you can change to any mod at any time.
The arrows are pointing to the box at the bottom of this page. This box, or a similar one, will appear when the mod you have chosen to view has multiple pages which explain new items (i.e. tools or blocks) that the mod introduces into Minecraft.
By clicking anywhere on the Lime Green bar (or on the word 'show' to the right side of the bar) with the name of the mod you wish to view, the items will be revealed, and you can click on any of the available names to find out more about that item.