PubMed Central (PMC) is a free digital archive of biomedical and life sciences journal literature at the U.S. National Institutes of Health's National Library of Medicine (NIH/NLM). As of May 2020, PMC contained more than 11 million full-text articles from over 4,600 journals. PMC is part of PubMed, a larger database containing 27 million citations and abstracts for biomedical literature from around the world.
The process for getting a PMCID can vary depending on the journal in which you publish. Some journals will automatically assign a PMCID to your article upon publication, while others require that you submit a request form along with your manuscript. The NIH/NLM provides detailed instructions on how to request a PMCID, which you can find here: https://www.ncbi.nlm.nih.gov/pmc/about/requesting-pmcid/.
Generally speaking, it can take anywhere from 2-6 weeks to receive a PMCID once you have submitted all of the required information and materials.