A Collection Score is based on the value of cards you have in your collection. What each card is valued at can be found here.
Your Collection Score determines your "Collection Level " which qualifies you for a number of rewards. These rewards are balanced in a way that means you will need to deliberate whether you wish to keep you cards that contribute to your score vs selling the card on the market to generate coins to purchase alternative cards you wish to chase.
Your Collection Score and Collection Level are only for the current active season. At the start of the new season, your score and level is reset and new rewards become available.
Your Collection Score and Collection Level are both shown inside your card collection and show the rewards available for the current season and the score required to get those rewards.