Note: The "Issues" label is often customized and may be called something else in your customer instance, like Project Log or Issues/Risks.
Issues allow users to log, track and resolve project-related topics such as problems, change orders, bugs, requests, ideas and general issues. Each issue is assigned to a resource, and has a set of fields - the exact fields are determined by your organization. For a list of all out-of-the-box (standard) fields and their definitions, see Project Logs/Issues: Out of the Box Fields.
Issues can be accessed from Home/Issues, the Issues navigation icon, as well as from the Issues section for each Project. See:
Working with the Project Logs List page
Profile-Based Permissions for Issues/Project Logs