Postel's Law or The Robustness Principle, is simply defined as "be conservative in what you send, be liberal in what you accept"

The application is, whenever machines need to talk to one-another, that they should accept messages that might not be perfectly formatted but the intention/message is clear, but should be very rigid in what they send.

If you send a message, you should ensure you conform completely to the specification/standards that governs your message format (JSON for example) but be flexible when receiving messages and ensure you don't crash/error if the input is malformed to you.