What does deduction 1 mean?

It means that you can reply to the message with the content "1".

Generally, it is a simple word used to respond to "yes" or "no", indicating that the above content has been received, agreed, and supported. The details will depend on the announcement.

Extended information:

When the information publisher publishes a message in the group, the content of the message is usually "Deduction 1 for receipt", and "Deduction 1" is also effective and convenient. Information publishers can also clearly know those people who have received messages and those who have not received the messages based on the statistics of message recipients, thereby improving work efficiency.

Reference: Baidu Encyclopedia - Terminology