Marketplace Partners provide apps and integrations available on the Atlassian Marketplace that extend the power of Atlassian products.
July 1, 2020 edited
ACP-600 Project Administration in Jira Server is a great place to start as it gives you a good grounding in using Jira without you needed a couple of years of practical experience as a Jira Administrator to call on.
In the last couple of years Atlassian University has made more and more of their training free. Check out the Jira Project Administrator learning path as an example.