In Discourse, users mostly gain trust by reading posts. As long as there aren’t enough posts to read, users cannot gain trust and would stay at trust level 0, which severely limits the speed at which they can create posts.
To help a new install get started quickly, bootstrap mode boosts new user’s initial trust level to 1 as long as there are less than 50 members (and for an additional 24 hours after that). After that, you hopefully have enough posts for users to gain trust level 1 naturally 
Additionally, daily email digests are enabled for these users so they are pinged to return to the site a bit more urgently. Once bootstrap mode is turned off (which happens automatically once you have more than 50 members), this digest setting is reverted to its default.