How can I use categories to filter entries?
The relatedTo parameter requires one of the following as an argument:
- AssetFileModel
- CategoryModel
- EntryModel
- UserModel
- TagModel
- An element’s ID
- An array of element objects and/or IDs

Passing in a string from the URL will not work. If you are not on a template that is the result of a category route, you need to fetch the category first.

{% set thisCategory = craft.categories.handle(lastSeg).first() %}


You can then use thisCategory as your argument in the relatedTo parameter.

{% set meubels = craft.entries.section(‘meubels’).relatedTo(thisCategory) %}