Learn Finnish: Kuntoutus on paras voimavara, kun tuemme syrjäytyneitä. - Rehabilitation is the best resource when we support the marginalized.

It is a participle acting as an adjective, and here it is used independently as a noun to mean "the marginalized". Just like in English, Finnish allows you to drop the head noun (like ihmisiä) and use a plural adjective to represent a whole demographic group.

Why is syrjäytyneitä in the partitive plural instead of the nominative plural (syrjäytyneet)?

Because it is the object of the verb tuemme (we support). The verb tukea (to support) is a partitive verb, meaning it always requires its object to be in the partitive case.

What kind of word is syrjäytyneitä originally? It looks like a verb form.

You are right, it is the active past participle (the NUT participle) of the verb syrjäytyä (to become marginalized or socially excluded). These participles frequently act as adjectives, which means they can also be treated as nouns to describe groups of people who have undergone that action.

What are the parts of the compound word voimavara?

It is made up of voima (power, strength, force) and vara (reserve, fund, provision). Combined, they create the word for "resource" or "asset".

The superlative of hyvä (good) is paras (best). Could I use here instead?