User Filters are used to add one or more filters to target customers. After the automation trigger and Trigger filters are set, the next step is the User Filter. This filter is set based on user data.
For example, in the Automation Filter, you've chosen Checkout started and on the Trigger Filters, you choose Subtotal Price and then Greater than 200. And for the User Filter you've selected Customer ID > equals 000001. So, now whoever has this Customer ID will receive the automated texts.