What does speed do?

Speed is the stat the attributes to how often your characters go, a lower speed character could go before your yoda if they have a leader that gives them more speed or if someone hit your yoda and removed turn meter.

But if you really want to know more about how speed works these guys at team instict did a pretty comprehensive overview of speed a long time ago.

http://www.teaminstinct.net/swgoh-guide/speed-and-you-a-team-instinct-special/