Row filters are a way to filter the rows in a slice. Below, you can see the expression in blue next to the "Row filter condition" label. This expression defines the Yes/No filter condition used by the slice. Rows that satisfy the condition are part of the slice; those that fail the condition are excluded.
When you click the blue expression, Expression Assistant is launched. Use Expression Assistant to construct an appropriate filter condition.
Filtering Rows by User
You can choose to filter data based on the user currently using the app. In order to do this, you first need to make sure your app requires sign-in, so AppSheet can recognize who is signed in to the app and render the corresponding data.
You'll also need to make sure that your spreadsheet contains a qualifying column that can uniquely identify the user based on the row. The contents of that column should identify the user. Email is always best, but you could use Name, Student, User, etc.; anything that will require the user to type in the qualifying detail.
The row filter condition [Student Email] = USEREMAIL() will ensure that only the appropriate rows are included.
Note: avoid USERNAME() because it is unreliable. Providers like Google only return the user's name if that user has enabled Google+ on their account. Instead, use an Email column and compare it with USEREMAIL(), rather than comparing a Name column to USERNAME().