Please visit https://learn.microsoft.com/en-us/office/dev/add-ins/overview/set-up-your-dev-environment?tabs=yeomangenerator for initial environment to develop an Office add-in.
After your environment is ready, please follow below steps to run the sample:
git clone https://github.com/OfficeDev/Word-Writting-Assistant-Add-in-Sample.git
npm install
npm start