A conversion is a user action that you consider to be of direct value to you.
In the case of e-commerce tracking, establishing what counts as a conversion is easier. It’s usually the sale of a product, but it could also be the number of “add to carts”, use of vouchers, clicks of a certain button etc.
For other websites, a conversion can be a totally different thing: a sign up, accessing a key page, filling in a questionnaire.
In order to make full use of the analytics data, especially when you have explicit marketing objectives, you have to decide what a conversion is to you, at a given time. Then, you should think about having a landing page associated with that.
For example, online stores usually have a “thank you for your purchase” page that shows up at the end of the acquisition process. When this page loads, you have a sale. Pageviews on this part of your website can be counted as conversions. Make sure to track this specifically and make use of it to understand what percentage of users actually become customers.