Allowing users to filter the records in the portal on their own gives them the ability to easily navigate through their records in their portal especially if there are lots of them. To set this up, you need to go to:
Portal Tables
Click on the linked records field where you want to set up the filter.
Go to the Layout section.
Client-side Filters
Turn on Allow users to filter records.
Click Add field button and choose the field.
You can add many fields that your users can use in their filters but if no fields are added, it shows all the fields. This toggle is turned on upon creation of the portal. On the portal, the filter button will become available.
💡Note
The dropdown filter option is available for single and multi-selection fields, while the input field for the filter changes based on the field type you are using for filtering.