What are attribution models?
Attribution models are used so that you can properly credit sales and conversions. They allow you to determine where the user clicked right before they converted. This is another way to follow the user experience, see patterns and determine what is working to create these conversions.
What one to use
Google provides default models to chose from, or you can create a custom one. When deciding what you want to credit each conversion in different scenarios you have to think about the different user flow that may happen in the process. For example, if you use the First interaction model, the credit would be given to what a user first clicked on to get to the site when they converted. The Last Interaction model credits the last thing that a user clicked on before the conversion. There are other default models that google provides or you can create a custom one. To read more on the specific default models check out this google support page.