An investigation should be a thorough inspection of the situation in order to collect as much evidence as possible, so that the employer can make an informed decision about how to proceed. In a harassment complaint for example, this will mean interviewing the complainant and any witnesses, and then giving the accused the opportunity to thoroughly understand the allegations against them so that they can respond.
Investigations come in all shapes and sizes. While it can mean an extended process involving dozens of witnesses and hundreds of documents and culminating in a lengthy written report, it can also mean brief discussions with two people followed by a concluding email. That said, every investigation should include an opportunity for the “accused” to give their side of the story. It must also include a clear conclusion: even if it is a “he said, she said” situation, an investigator has to weigh the evidence and decide what they think happened.