github

how to use github files and how to use like this “mbedtls / md.h”

You need to download the .h and associated files from GitHub and install them. There is often a zip file of the complete library available on GitHub that you can download to your PC. If that is the case then after doing the download look at Sketch->Include Library->Add .ZIP Library in the IDE

Note that using " " around the library name in the #include instead of < > will cause the compiler to look for the library files in the sketch directory first before looking for it in the libraries folder of the sketchbook folder

As a side note, please post the text of programs here rather than a picture of it

Please post a link to where you found this code.

Which Arduino board are you using?