It means that the client will only send that cookie through a secure (HTTPS) connection. This means you'll have to forward the user to a secure URL in order for the cookie to get sent to the server.
You can set a secure cookie through an insecure connection, though you obviously should not (otherwise the value of the cookie may be sniffed). Since a secure cookie can be changed by an insecure connection, you cannot trust that the cookie value was not corrupted by a third party that intercepted and changed the contents of an insecure HTTP request to your site. Therefore, depending on how you're using the secure cookie, you may need to validate its contents.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…