An abandoned cart is the result of a potential customer who has added an item or items to their shopping cart but does not complete the purchase process.
Although some abandonment is unavoidable (think of it like the ecommerce equivalent to window shopping), it's important to track your abandonment rate to get an understanding of where your storefront experience has too much friction.
One of the common places for abandonment is at checkout - also referred to as "Checkout Abandonment Rate" - and can be caused by a number of reasons. For example, there may be certain details presented to the shopper at checkout that discourage the purchase (ie delivery times, shipping costs, or return policies) or the checkout process could be too complicated (ie requiring the shopper to create an account).
You can likely reduce abandonment rates by experimenting with your shopping and checkout process through A/B testing, remapping your customer journeys, or providing more and clearer information on your PDPs.