Learn Finnish: Arvoisa yleisö, tervetuloa! - Esteemed audience, welcome!

Even though you are welcoming an audience, tervetuloa is a fixed greeting rather than an active verb. It is made of terve (healthy) and tuloa (the partitive form of tulo, meaning arrival). Because it is a set noun phrase rather than a verb, it never takes an imperative command ending.

What exactly does arvoisa mean?

It translates to esteemed, dear, or honorable. It comes from the noun arvo, which means value or worth. Adding the ending -isa turns it into an adjective. You will hear it often at the very start of formal announcements or speeches.

Why is arvoisa yleisö in the basic dictionary form if we are addressing them directly?

Finnish does not have a special grammatical case for addressing someone directly (which some languages call a vocative case). When calling out to a person or a group, you simply use the basic nominative form.

Is yleisö a singular or plural word here?

Yleisö is grammatically singular, even though it translates to audience and refers to a large group of many people.