Soft counter is a counter-unit or counter-ability that is general between economic-counter and hard-counter in effectiveness.
Example: Tank Destroyer vs Tank. Tank Destroyer will suffer damage from Tank, but is superior unit overall.
vs Hard Counter - Soft counter is usually of same type/class as countered units and will sustain some damage.
vs Economic Counter - Soft counter is able to stand the ground in 1v1 attack, usually with more/specific armor or defensive stat vs the enemy unit.