Serve resources from a consistent URL

GTMetrix is reporting the following:

Serve resources from a consistent URL
https://www.XXXXXx.net/wp-json/contact-form-7/v1/contact-forms/1826/refill
https://www.XXXXXx.net/wp-json/contact-form-7/v1/contact-forms/38/refill
https://www.XXXXXx.net/wp-json/contact-form-7/v1/contact-forms/9332/refill

The above links just has the following text:
[]

Any ideas what these files why do they exist?

Thanks