It is as you say: your post has been edited.
Every question and answer on the site has a revision history, which shows each version of the post and provides comparisons between them. This is the revision history for a question of yours that was recently edited. It sounds from your description like you're talking about that page.
The revision history shows the edits that have been made to your post, by yourself or others. Text that is highlighted in red and crossed out is text that an edit removed. Text that is highlighted in green is text that an edit added.
If you are wondering why your post has been edited at all, then Why can people edit my posts? How does editing work? in the help center (as Kulfy mentioned) explains this. Although Ask Ubuntu doesn't work quite the same as a site like Wikipedia, posts are editable and edits from anyone are generally encouraged. This applies both when the editor is the same person who created the post and when they are different people.
Although the system has informed you of the edit, no specific action is expected from you in connection with it. The color highlighting and strikethrough shown in the revision history is just a way to show the changes that were made. This isn't telling you what changes you should make; it's telling you what changes were made.
With that said, if you think the edit is in error, then you can edit the post again (you can even roll back the edit from the revision history). In this case, the edit added information that is relevant to your question and that you provided in comments. It also removed some very general introductory text and some signature-like text at the end, which weren't really helping the post. As far as I can tell, this edit was helpful; I don't think it introduced any errors.
Sometimes, someone will suggest an edit on one of your posts, but that edit won't yet be applied. This happens when the editor doesn't have enough reputation to apply edits directly, so their edits must go through review. The amount of reputation required to edit without going through review varies, but for most kinds of edits, it's 2000. When a user with less rep edits, they are actually submitting an edit suggestion.
When that happens on one of your posts, the system will notify you of the edit suggestion. In that situation, the proposed changes, which have not yet been made, are shown, in a manner similar to what is shown in the revision history. You can decide to accept or reject the edit.
But even in this situation, you don't have to take any action if you don't want to, because suggested edits also go into a review queue where they are reviewed by other users. In fact, most suggested edits are accepted or rejected through the review process, rather than by the owner of the post. This is mainly because there are people from all over the world reviewing suggested edits all the time, and they'll often get to it before you do.
As with any edit, whether by yourself or by someone else, you can always later make another edit to improve upon it further or to correct anany errors it may have introduced.
