This is the number of seconds a notification will be held by the Web Push providers (e.g. Google or Apple) before being delivered to your subscribers if their device is not online by the time of sending. If the user does not come online within the provided time, the notification will get removed and will not show to the subscriber.
In each type of campaign there is the option of time to life. For most notifications campaigns we recommend to use a time to life of 1 week, which is the default in our platform. The maximum TTL is 4 weeks (2419200 seconds).
For example, if you set TTL to 10 hours (36000 seconds) and my device is offline for 10 hours and 1 second or longer, I will not get the notification.