This is the 614th most frequent Finnish word.
tärkein
Most important.
Here, 'tärkein' is the superlative form of 'tärkeä' (important), used as an adjective to describe 'asia' (thing), conveying the meaning of being the most important of all.
Tärkein asia elämässä on perhe.
The most important thing in life is family.
In this sentence, 'tärkein' describes 'ratkaisu' (solution) and indicates that it is the solution of utmost importance.
Tavoitteemme on aina löytää tärkein ratkaisu.
Our goal is always to find the most important solution.
Tärkein is used as a superlative adjective emphasizing the most important aspect, which is the goal in this context.