Learn Polish: Wczoraj szef przeczytał twój raport. - Yesterday the boss read your report.

Because przeczytał is the perfective form of the verb. It emphasizes that the boss completely finished reading the report. If the sentence used the imperfective czytał, it would mean he was in the process of reading it yesterday, but might not have finished.

Since twój raport is the object of the sentence, why didn't its ending change?

Raport is a masculine inanimate noun. In Polish, when a masculine inanimate noun is the direct object (using the accusative case), it looks exactly the same as its basic dictionary form. The pronoun twój matches this, so neither word changes.

How does the verb przeczytał show who is doing the action?

In the Polish past tense, verbs must match the gender of the subject. The -ł ending on przeczytał tells us the subject (szef) is masculine singular. If the boss were a woman (szefowa), the verb would change to przeczytała.

Is it okay to put wczoraj at the end of the sentence instead?

Yes, Polish word order is flexible. You could say Szef przeczytał twój raport wczoraj. Putting wczoraj at the beginning just slightly emphasizes that this happened specifically yesterday.