By default, b2evolution will only display the "Teaser part" of a post while browsing a Blog/Collection and will display the "full post" (complete text) when clicking on a post title or "read more" link. (However, if the "No Teaser" checkbox is checked, then the "full post" view will not include the teaser.)
[teaserbreak] effectively creates a difference between the "teaser" part of a post and the "full post". If there is no
[teaserbreak] in a post, then there is no difference between the "teaser" part and the "full post"; in that case, b2evolution will always display the "full post" instead of a teaser.
Note: Each post also has an Excerpt, which is a very short summary/excerpt of the post. The excerpt is typically much shorter than the teaser.