In the Speaker Page of Community, all the Speakers are displayed in First Name, alphabetical order.
For each Speaker within your Community, you're able to assign one or more Sessions to the Speaker, so they appear in the Speaker List within that Session (more on creating Speakers can be found here). For Sessions with multiple Speakers, you may want to have them display in a certain order (for example, the order in which they are speaking).
Within Sessions, Speakers are displayed in the order in which they were associated with that Session. So, if you want to have your Speakers display in the order in which they are speaking, you'll need to add the first Speaker to the Session first, followed by the second, and so on and so forth.