The difference between mikä and mitä

Both mikä and mitä mean "what?"

1. Mikä

When using mikä, you are referring to a defined, concrete thing, something countable. You use it when you want to know what something is, mostly with the verb olla.

Finnish English
Mikä tämä on? What is this?
Tämä on pöytä. This is a table.
Mikä tuo on? What is that?
Tuo on tuoli. That is a chair.
Mikä tämä on? What is this?
Tämä on jauheliha. This is minced meat.

2. Mitä

Mitä is the partitive form of "mikä". When using mitä, you are referring to something abstract, something undefined.

You mostly use mitä with more active verbs than "olla". If you do use "olla", it's usually to find out what something is made of (pork, lamb) rather than what exactly it is (minced meat).

When the verb in your sentence is a partitive verb, it makes sense that the questionword is partitive as well.

Finnish English
Mitä tämä on? What is this?
Tämä on naudanlihaa. This is beef.
Mitä kuuluu? How are you?
Hyvää, kiitos. I'm well, thanks.
Mitä kello on? What time it is?
Kello on viisi yli viisi. It's five past five.