All issues found during a crawl will be prioritized as either high, medium, low, or notice priority in the Site Auditor, which can be visualized in the Issues by Priority chart.
Understanding issue severity is a crucial step in prioritizing SEO tasks. In most cases, it will not be possible (or necessary) to fix all issues found by the Site Auditor, so resources will need to be properly allocated to fixing the most important issues first.
High Priority Issues
Issues that should be given your focus first. The consequences of these issues frequently result in content not being available to visitors, content not being indexed by search engines, or a severe negative impact on rankings. Typically these should be fixed as soon as possible.
Medium Priority Issues
Issues in this category typically indicate under-optimized pages. The consequences of these issues are typically not disastrous but represent a lost opportunity. After fixing high priority issues, medium priority issues are often worth your time, especially if they are affecting high-value pages.
Low Priority Issues
These issues may be affecting the organic performance or user experience of your site, but are unlikely to have a major impact. These issues are usually very common and widespread across most websites, and sites can be very successful even without fixing these types of issues.
Whether or not these issues are worth fixing will depend on available resources, the priority of affected URLs, and other external factors.
Notices are areas of concern that deserve attention to ensure they are set the way that was intended. These are phenomena that are not inherently good or bad by themselves but instead depend on their implementation.
For example, Blocked by robots.txt is a notice issue. If a URL that you do not want to be crawled by search engines is blocked by robots.txt, then this is working as intended. But if a high-value page is unintentionally blocked by robots.txt, this could have severely negative consequences.