You can collect testimonials from your clients and share them on your sales pages and other places.
As an admin
To manually add them in the admin interface, first make sure they are enabled in the More menu:

Now navigate to Content > Testimonials.

Click on ‘Add a testimonial’ on the top right corner:

Include the author's, the testimonial, rating stars, the product it is related to, and the date.

Then click on create testimonial.
Turn On the Live toggle to make it visible to your clients on the testimonials page.

As your customer
You can now share a link to the Testimonial submission form.
Find your unique link on the Testimonials header, top right corner:

The testimonial form is located on your Main Website:

Once submitted, the admin will be able to mark it live, adding it to the Testimonials page.