How to use a text area field:

To use a textarea field, first drag and drop the form widget onto the page. Hover over the form widget and click on the pencil icon. Next, drag and drop a basic textarea field. Double click on the field and click the pencil icon to edit it. 

A textarea form item allows you to enter long text comments into a form and submit them. A text area field is a field in a form where you can insert a block of text rather than a single line. The text area field serves as a place to fill in short descriptions. This item is a form item and can only be added to a form.

Here is how to use a text area field:

Step 1; First, drag and drop the form widget. Next hover over the widget and click the pencil icon.
Step 2: Drag and drop a basic textarea field (A to B).
Step 3: Double click on the field to edit it (B).

You can make this field a required field for your visitors to complete by ticking the required box (C) and increase or decrease the text area by adding rows (D).
Eventually the email that will be sent after submitting this form will include the text which was inserted in that text area field.