Allowing users to filter records in the portal on their own gives them the ability to easily navigate through their records, especially if there are lots of them. This option is enabled by default for new portals. To enable it on an existing portal, you need to go to:
Portal Tables.
Click on the table
Go to the User Actions section.
Enable the Filter button toggle in the Client-side Filters section.
If you want to also enable filtering using dropdown filters, you can enable the Dropdown filters toggle and select which single or multiple select fields to use as dropdown filters in the Dropdown Filter Fields list.
Here's how the button is displayed within the portal:
