When you place a graphic, you will see a screen-resolution version of the file in the layout, so that you can view and position it, but the actual graphic file may be either linked or embedded.
Linked artwork is connected to, but remains independent of, the document, resulting in a smaller document. You can modify linked artwork using transformation tools and effects; however, you cannot select and edit individual components in the artwork. You can use the linked graphic many times without significantly increasing the size of the document; you can also update all links at once. When you export or print, the original graphic is retrieved, creating the final output from the full resolution of the originals.
Embedded artwork is copied into the document at full resolution, resulting in a larger document. You can control versions and update the file whenever you like; as long as the artwork is embedded, your document is self-sufficient.
To determine if artwork is linked or embedded, or change its status from one to the other, use the Links panel.
If the bitmap image you place is 48K or smaller, InDesign automatically embeds the full‑resolution image instead of the screen-resolution version in your layout. InDesign displays these images in the Links panel, so that you can control versions and update the file whenever you like; however, the link is not necessary for optimal output.