As a Basic (i.e. free/unpaid) member, you have the ability to read and respond to members who message you.
When you receive a message from another member, click on the message that you received from within your Message Inbox and choose the option to "Start Unlocking". When you do this, it'll begin a countdown until the contents of the message are revealed to you. At the end of the countdown, return to that same message in your Message Inbox to view its contents.
To bypass the countdown and read your message immediately, purchase a subscription at https://www.catholicmatch.com/subscribe