SplinterGlossary: Combining Cards
Combining cards is the mechanism that allows them to become more powerful, grow stats and unlock new abilities. That's the case for monsters. For summoners, by leveling them up, one can play monsters at higher levels.
When reaching or exceeding certain BCXs levels, a card is leveled up. The lower-level cards that are combined are burned and a new card is created that is the combined version of those cards.
The BCX of the combined card grows to reflect the total BCXs of the lower-level cards.
If you combine two 1 BCX cards, you obtain a 2 BCX card (and you no longer have the 1 BCX cards).
For regular common cards from the Chaos Legion set, if you combine 2 x 5 BCX versions and 4 x 1 BCX versions, you end up with a 14 BCX card, which for a regular common Chaos Legion card means upgrading the card to level 3.
You can check the needed BCX to level up to different levels and the benefits associated with that level from any card's stats page in Splinterlands. The requirements differ based on rarities, on foil, and even on the edition.
Combining cards is irreversible! It's very important to not combine more cards than needed to level up a card. The remaining lower-level cards can always be put on the rental market until there are enough copies to level up more (if that's the goal).
There is a max level upon which cards can be leveled up. Combining cards beyond that level is a waste.