OP_RETURN is a script opcode which can be used to write arbitrary data on blockchain and also to mark a transaction output as invalid. Since any outputs with OP_RETURN are provably unspendable, OP_RETURN outputs can be used to burn bitcoins.
How to use OP_RETURN with Trezor
1. Click on Add OP_RETURN in Send transaction tab.
2. Write any text in the Data (text) box.
3. Click send. Your text is now permanently written in blockchain.