How the bookmarks work

Bookmarks (sometimes also referred to as markers) have been introduced to allow quick tuning of predefined frequencies, and as a way to allow users to qickly identify transmissions without the need to be familiar with the receiver's location and HF circumstances.

They're the little colored "bubbles" that show up above the frequency bar on the top. They can come from three separate sources, which correspond to the color of the bookmark:

Bandplan frequencies (FT8 & co) in green, common amongst all users. Some specific modes have been tied in to specific frequencies, these are kept in the bandplan (bands.json) and are subject to little changes over time. The bandplan can be modified by the receiver operator.
Server-side bookmarks in yellow, common amongst all users. These are the classic bookmarks where you'd put your repeaters and radio stations. They are kept in a file named bookmarks.json which can be modified by the receiver operator.
Client-side bookmarks in light blue, local for the current user only. These bookmarks can be created using the bookmark button on the receiver panel and are stored in the browser's localstorage. They are not sent back to the receiver and can be modified by the user.

File location

The location of each of the files mentioned varies with the type of setup. In most cases, they should be kept in /etc/openwebrx/. If you have setup your receiver from source, the files will be located in the checkout folder, but it is recommended that you copy them over to /etc/openwebrx/ and maintain your changes there. You will probably need to create the directory initially.

File location when using docker

If you are running OpenWebRX inside a docker container, the file will be kept in /etc/openwebrx/ as well, but that is in the container's filesystem. In order to maintain this file, you can mount a volume or directory into this location and then edit the files there. The docker container will copy its initial files to that location if it doesn't find anything on startup.

Directory mount: -v /my/local/directory:/etc/openwebrx - you will find the files in /my/local/directory after the first run. For consistency, you can use -f /etc/openwebrx:/etc/openwebrx - that way your files will be in /etc/openwebrx on the host and inside the container. You will probably need to create the local directory first.
Volume mount: -v openwebrx-config:/etc/openwebrx - you can find out where the files are stored on your local filesystem by running docker volume inspect openwebrx-config, they are usually under /var/lib/docker/volumes/openwebrx-config/_data. You will probably need to create the volume beforehand: docker volume create openwebrx-config