The menus and other items grouped together on the left are each in a 'Block'. Blocks can be 'collapsed' or closed and 'expanded' or open.

Those triangles are there to let you know if a 'Block' is collapsed or expanded. If it points down, then it's expanded. If it's pointing left at the title, it's collapsed.

Click anywhere on the Block title and you will 'collapse' it. If you want to see less text on your screen, collapse some Blocks. Click again to expand. When you're logged in, the website will remember your Block preferences.