‘Securing your spot’ just means that you’ve accessed the challenge before it’s been released - it’s like you’ve signed up for the ‘early bird’ or ‘pre-release’ version of the challenge. Your trainer may still be putting the finishing touches on some of the workouts and exercise tutorial videos at this point, so it’s not ready fully for the community just yet!
Once you secure your spot, you’ll see a panel on your calendar that counts down until the challenge goes live. Furthermore, you’ll receive a notification when the challenge actually goes live, allowing you to schedule it in.
Once a challenge goes live, this is when it’s actually released - and at that point, the challenge is completed, fully ready and you can schedule it into your calendar, ready to go.