It means "would be". So Miksi tämä olisi väärin? would mean "Why would this be wrong?"
It's the conditional mood of the verb olla. You take the root ol-, and add the conditional suffix -isi.
This course also uses this conditional mood on voi, "can", to make voisi, "could", and on haluatte, "you want", to make haluaisitte, "you would want/you would like".
Tee is an imperative verb, a command. The object of an imperative verb either looks nominative like se, or is partitive like sitä. Tee se is a complete sentence, with only one noun.
Tehdä is an infinitive verb. The object of an infinite verb is either accusative like sen, or partitive like sitä. Tehdä sen would not be a complete sentence; an infinitive verb needs another verb.
This other verb may have a noun as its subject, like Hän haluaa tehdä sen. So this sentence has two nouns, and while the subject is normally nominative, the object is either accusative or partitive.