Before a scientific paper is published in a journal, it is usually peer-reviewed. The aim is to make sure that the research is reliable and credible.
During the peer-review process, the paper will be vetted by a group of scientists who will assess if the science – the method, the analysis and the inferences drawn from the data – stands up to scrutiny. This is designed to weed out errors, misinterpretation or flawed research methods.
A 'pre-print' is a paper that has not yet been peer-reviewed or published in a journal. The peer-review process takes time, so in order to speed up the distribution of research, scientists sometimes post papers to pre-print archives before they are published in a journal.