It means the grey lines are in your private zone and others can only see the red part of your activity.

Strava hides the start and end points of every activity, to protect user privacy. I believe it’s ⅛ of a mile around the start and end by default, and it affects which segments your activities qualify for.

The gray GPS track is the part of your activity that is only visible to you, while the red section is visible to everyone who can view your activity. In other words, someone viewing your activity would only see the red section.

You can modify this behavior by going into your Privacy Controls and changing the following settings 👇

If you choose to disable Hide the start and end points of activities no matter where they happen, you are advised to advised to hide the start and end points of activities your home/work/any other sensitive locations.

For more information, see Strava’s support articles.