Your Strava ID is a specific number assigned to your account when you register on Strava. Your ID can be found in the address bar when visiting your profile page in a web browser. To get to your Profile, log into the Strava website and click your profile picture in the upper right-hand corner or left-hand side. Then, look at the browser's address bar for the number at the end of the URL.
For example - https://www.strava.com/athletes/7174722