A conversion happens when someone clicks your ad and then takes an action that you defined as a conversion goal, such as an online purchase. With conversion tracking, you have the flexibility to count conversions according to your business needs. For any conversion action, you can choose to count all conversions that happen after a click, or only unique conversions that happen after a click.
These counting choices can give you a better sense of your campaign performance and help you refine your bids.
Example: You track two conversions: leads and sales. You pick "Unique” for leads and “All” for sales. When one ad click turns into two leads and two sales, it's counted as three conversions: one for the unique lead, and two for all the sales.