Learn Finnish: Pääovi on lukittu. - The main door is locked.

No, it describes a resultative state. It means the action already happened, and now the door is currently in a locked state. It is formed using the verb olla (here as on) plus the passive past participle lukittu. If you wanted to describe an ongoing action ("the door is being locked"), you would use the passive present: pääovi lukitaan.

What is the dictionary form of the verb used to make lukittu?

The base verb is lukita (to lock), which is a Type 4 verb. To make the passive past participle form that we see here, we use a specific ending (-ttu/-tty or -tu/-ty). For lukita, it becomes lukittu.

What does the pää part of pääovi mean?

On its own, pää means "head" or "end". However, when used as the first part of a compound word, it usually means "main" or "chief". So pää + ovi (door) becomes "main door". You will also see this in words like pääkatu (main street) and pääkaupunki (capital city).

I've learned that "lukossa" means locked. Can I also say Pääovi on lukossa?