The Azure Security Center is now generally available as announced in Azure Security Center, now generally available, proven to improve security. Cybersecurity, including threat detection and analysis, is challenging and requires specialized skills, training, and resources that might not exist in an organization or could be too costly to implement. Though not a replacement for skilled analysts, Azure Security Center provides features to help address the challenges of detecting potential attacks and analyzing environments for potential vulnerabilities.
The Azure Security Center page is the best starting point with links to capabilities, documentation, and pricing.
In regards to pricing there are two offerings: free and standard. In brief, both offer analysis and basic security alerting while standard offers more advanced threat detection including behavioral analysis and anomaly detection.