It is a general and versatile exclamation. It comes from the verb tetszik. Actually, it is the imperative 3rd PS indefinite form of tetszik. Its meanings can be:

If someone is knocking at the door:
Come in! – Tessék!

If you give somebody a gentle order:
Please sit down. – Tessék csak leülni!

-May I come in? -Bejöhetek?
–Please. – Tessék!

If you didn’t understand something:
I beg your pardon? – Tessék?
> In this case it is a question.

When sitting at the table:
Help yourself. – Tessék!