Testimonials are similar to product reviews, except they are about your whole store and not specific to a single product.
Testimonials are essentially store reviews.
In order to show your testimonials, you need to add a testimonial widget to your site.
How to Add a Testimonials Widget
Go to the Widget section on the left-hand side of your dashboard.
Click the + Add Widget button
Select Reviews
Under 'where to show reviews', choose Homepage
Select Store Reviews Only for what type of review you want shown
You will then be taken to the widget editor to configure your widget before launching it.
Go to Content
Under 'Let Me Choose', select Change
Then click on Show All
Once you have configured your settings, click Save Draft
When you are ready to show your testimonial, click the green Launch button