Block mode is a web application firewall (WAF) setting in which a rule that matches a request takes action to stop it, for example by returning an error page or dropping the connection. The alternative is a log, passive or detection mode, where the WAF records the match but still lets the request through. Different products use different names for these settings, so check how your own WAF describes them.
Teams often start new rules in log mode to see what they would catch and to avoid blocking legitimate traffic by mistake. That is sensible, but it means a rule can exist, appear in a dashboard and even generate alerts while providing no protection at all. A WAF that is turned on is not the same as a WAF that is blocking.
Block mode also does not guarantee that a specific attack is stopped. Protection depends on whether a rule covers the particular technique, whether the traffic actually passes through the WAF, and whether the attacker can vary the request to avoid the rule. A rule that blocks one form of a request may not block another.
To check your position, confirm the action configured for each relevant rule, review the logs to see whether matching requests were blocked or only recorded, and test with safe, non-harmful requests in a controlled way. Treat the WAF as an additional layer and still apply vendor patches to the underlying software.