As I understand it, the mortgage is reviewed on an annual basis to ensure that the repayments are set at the correct level in relation to the Euribor rate.
For example if, at the beginning of your mortgae you pay €500 per month for a mortgage at Euribor +0.5%, when Euriobor is say 4% then that has been calculated to repay the mortgage or cover the interest charges, whichever the case may be.
However, if say after 3 months, Euribor increases to 5% then the €500 is not going to be sufficient to cover the interest/capital.
It is similar to how the B/Socs in the UK used to do things a few years ago in respect of variable rate mortgages, which in effect a Euribor mortgage is.
Noreen