Verb: to be hurt, to be injured, to be wounded, etc. (Many additional meanings, so check the sanakirja.org definition for more.)
Loukkaantua appears in our example in the 3rd person singular past tense form loukkaantui.
Loukkaantua is derived from
Let’s translate the whole headline, 13 ihmistä loukkaantui neljän auton kolarissa Valtatie 1:llä or ”13 people were injured in a four-car crash on highway 1.”
Notice in the English translation the plural noun ”people” is used and yet the verb in Finish is still singular because the noun in Finnish (”ihmistä”) is singular. (Ihmistä is the partitive singular of ihminen (”person”), and the partitive singular is used because it comes after a number that is not 1.)