When posts are displayed on the DC homepage they only show the first part of each post. This part is known as the teaser, or the trimmed version. The DC software does its best, but without being given any hint by you it can sometimes produce teasers that are of limited value, e.g. "Hi!"
When composing your post, the Preview button lets you see what the teaser will look like. For example, here's the preview of an initial version of this post:
And if you want to take control, put the cursor at the point you want the teaser to end, then click this button on the editor toolbar: