Pop-ups contain information about features and images in map layers, such as hiking trails, land cover types, or unemployment rates. Pop-ups can include attachments, images, charts, and text, and they can link to external web pages.
The default pop-up appearance for a feature layer—if the layer owner has not configured it—is a table of attributes and values. The layer owner can save a new configuration, which is why the pop-ups in a map can reveal interesting information with no effort from you. The default pop-up appearance for an imagery layer is the pixel values.
Tip:
You can click Dock 
on the pop-up to dock the pop-up display to the top of the map. Click Undock 
to display pop-ups near their associated features or pixels.
For a particular map, the map owner can reconfigure the feature layer pop-ups. Map authors define the list of visible and hidden fields and how the information is presented. For example, they can show a simple list of attributes, or provide an interactive experience using custom-formatted text, charts, and images.
To learn more about designing and configuring pop-ups for feature layers, see Configure pop-ups.