Ignited
Fire has taken one of your blocks, and it can't be matched while it burns. Swap it and you smother it back into play at the cost of some HP, otherwise it burns itself out over time.
In detail
An ignited block cannot be matched. Swapping it puts the fire out and returns the block to play, but costs you HP, typically 2 to 5 depending on the enemy. The cost is charged per block, so a drag that passes through three ignited blocks bills you three times. The fire also goes out on its own when its timer reaches zero, which costs you nothing. Enemies place it two ways: an aura drips new ignited blocks on the enemy attack interval, and on-hit ignites roll separately on each strike that connects.
More in glossary
From the current development build. Content and balance may change before release. This public reference includes discoveries and does not reflect your personal unlocks.