I have a iphone 4s and its locked to telstra? What does that mean?

Does it mean i have to buy a telstra sim card or it won't work for any simcards and i want a plan!

Basically yes. When any phone is 'locked' it means that you can only use the network it is locked to. This usually happens when it has been originally purchased through a contract on that network (in your case 'Telstra'). However, if you call up the carrier they can usually unlock it for you, for a fee. In the UK the fee is usually £5/10, and once it's unlocked you can use any carrier you so desire.

But your phone being locked definitely shouldn't mean that it won't work with any carrier. The reason carriers lock phones in the first place is to try and encourage you to stay on their network so they can make more money, so not being able to use any network would be rather pointless for the money grabbing carrier.

That means the phone is locked to Telstra and can be used with Telstra only for now. If you unlock the phone, you can use the phone with any SIM you want. You can check online for factory unlocking the iPhone.