A registered agent is the person or company designated by law to receive service of process. “Service of process” is just legalese for handing someone a lawsuit or similar document. (Disappointingly, the process server doesn’t have to say “You’ve been served!”) Most, but not all, companies are required to designate such an agent and register them with the Texas Secretary of State.
You can easily search the Texas Secretary of State’s website for the registered agent for a minimal charge but you’ll need to setup an account.
But you can also search the Texas Comptroller’s site for free and with no login.
Some companies don’t have a registered agent list with either the Texas Secretary of State or the Comptroller.