Short answer
Carrier screening checks whether you carry a hidden recessive variant, like thalassemia, that is harmless to you but matters if your partner carries the same one, because a child could then inherit the condition.
Many healthy people carry a single copy of a recessive variant without any effect on their own health. It only becomes relevant when two carriers of the same variant have a child, who then has a one-in-four chance of inheriting the condition. In India, thalassemia carrier rates are high enough that screening is recommended in many guidelines before marriage or pregnancy. It is information for planning, not a judgement on health.