If you bought your DOTs before Polkadot went live, they will need to be transformed from DOT Allocation tokens on Ethereum (which represent a "right" to this number of DOT tokens) into actual DOTs on the Polkadot blockchain. That’s where the claims process comes in.

As part of this claims process, you first need to create a Polkadot account and then prove that you are both the owner of the Ethereum address and the owner of the Polkadot account. After proving that you have access to the DOT allocation tokens in the Ethereum address, the correct number of DOTs will be allocated to your account on the Polkadot blockchain.

You can find the step-by-step instructions on how to claim your tokens here.

There is no deadline to claim. Now that the Polkadot network is live, the claiming process will work indefinitely.