"Mapping" is the possibility of changing the order of the individual LEDs in the strip programmatically. Normally, all LEDs in a strip are connected in series (via the data line): 1->2->3->4->.... For some special effects or especially for 2D matrices you will often change the order without having to solder the whole strip together differently. For example, you can achieve that after each 3 LEDs the order of the LEDs is reversed, also by mapping, as if the logical order of the LEDs in the strip was like this: 1->2->3->6->5->4 ->7 ->8 ->9 ->12 ->11 ->10 etc.
Mapping is set via a JSON file, which must be named ledmap.json. You also need to prepare a special file in JSON format. For the example above, the content of this file will then look like this: {"map":[0,1,2,5,4,3,6,7,8,11,10,9]}. Please note that the LEDs are numbered starting with 0. You should also avoid using unnecessary spaces. You can omit individual LED positions by specifying -1 in their place.
Multiple maps can also be defined. For each map you then use a separate file called ledmapX.json, where X is the number (1, 2 etc.). Which map is then used can be specified in a preset with "ledmap":X,..., where X is the map number of the JSON file. In the simplest case, the content of the preset (API command) looks like this: {"on":true,"ledmap":1}. The ledmap.json file (even without a number), on the other hand, is not applied via preset, but always automatically when starting the controller.

The created ledmap.json file (or ledmapX.json files) must be uploaded to the WLED controller. To do this, open the address http://YourWLEDIP/edit in the browser, where YourWLEDIP is the IP address of your WLED controller. This is a tool to edit the configuration files on the WLED controller. In order for this page to be displayed correctly, your controller must be connected to the Internet, also already integrated into your WLAN network and no longer "only" available as a WLED-AP. You can also create the JSON files directly here. In order for the created or uploaded ledmap.json file to be applied, the controller must be restarted.