If you see this theme demo, you may be seen there is a section for clients testimonials.
You can add testimonials from the admin area. Navigate to testimonials > add new. Put the client name in the title box and the testimonial in the content box. Upload a featured image for client image. On the bottom side of the testimonial editor box, you can see the Client Details option box like the screenshot below. You can use these input box for details client information.
The testimonial itself will look like bellow in the front end of your site.