If you have played collectable card games before or even strategy games in general you already might have heard of this term. But what is value? A very basic explanation would be always trying to use your cards in the most efficient way that is available with the rule text it provides - but let's take a closer look.
Value is math. In the end everything in the game can be attributed to a certain value, from the stats of a unit over the damage of spells to the number of available resources or scrolls in your hand. Even the remaining life of your idols is linked to a certain value. Sometimes this equation is rather simple but more often the many influential factors make it very complex and still doing your math right seperates the good players from the bad ones.
As already mentioned above value means using scrolls in the most effiecient way the rules of the game allow them to be used. As an example the rules of Spark allow it to deal 2 damage to any unit for its cost of 2 resources and 1 scroll (Spark itself). Therefore, what you want to do is trade this for more than 2 resources and 1 scroll. That, for example, can be done by killing a unit (at this point you are equal in scrolls) with exactly 2 health but a very high attack resulting in a relatively high cost.
This is probably a good time to introduce the term cantrip - scrolls that make you draw scrolls when you play them efficiently replacing themselves and thus only costing resources. This can happen in form of units (Sister of the Fox), Enchantments (Languid), Spells (Kabonk) or linked to conditions like killing the target (Burn).
Keep in mind that not all value is instant. A strong unit can do a lot of damage over multiple turns generating high overall value or certain scrolls can make you draw more than 1 scroll over the course of the game.
Now you might think if this is all just math a perfectly designed bot could play this game better than humans. While this statement isn't even that far from reality (especially if you look at certain other online card games) you have to keep in mind that the ultimate goal is still just a condition and thus not linked to a number, just yes or no. If something would lead to you ultimately winning (or not losing) the game then it is the right thing to do no matter how many resources of scrolls you sacrifice for killing 1 unit or dealing 1 idol damage.
Also there is a psychological aspect to it. If you chose the lower value play to lead your opponent into a trap which allows for a win or higher value play next turn - then it obviously, in the end, was the higher value play.
Basically, what you want in your deck is a good mix of consistent scrolls that offer guaranteed value and more situational cards with potentially very high or low value - or even the possibility to become dead cards (having nearly no effect at all when used). Cantrips often offer consistency, same with rather high health units. Combo cards or high attack/low health units are more situational or risky plays. Finding the right balance is important and take your time to understand scrolls and how they can be used most successfully. More often than you think you will find a hidden value gem especially in the ever-changing meta where countering existing top tier decks is important. I hope this little guide gave a short insight on what all this value talk is about.
Feel free to contace me ingame or on scrollsguide for feedback or general help. Thanks for reading!