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What is a "payload adapter"? For example, from the following page:
The IXV Intermediate eXperimental Vehicle installed on its payload adapter, on 26 January 2015 at Europe's Spaceport in Kourou, French Guiana.
— IXV Installed on its Payload Adapter, ESA Space in Images
Payloads come in different shapes and sizes. To put them on a specific launch vehicle, you need to either design the payload to fit perfectly onto the rocket or you need some sort of adapter to mount it in place. It's also common to launch multiple payloads with a single launch, so you need some mechanism to separate the different payloads and release them into their respective orbits.
For more information, see the Wikipedia article on the ESPA
