Have you been living in the Czech Republic for a minimum of 6 months in the (current) tax year? You are usually considered as resident for tax purposes and have to pay tax on your worldwide income there.
Have you lived in the Czech Republic but for less than 6 months in the (current) tax year? You are usually not considered as resident for tax purposes and pay tax only on income earned in the Czech Republic.
Income from other EU countries? Check you never pay tax twice on the same income.