Literally, according to this reference in Louise Borden's The Greatest Skating Race: A World War II Story from the Netherlands (illustrated by Niki Daly; Margaret K. McElderry, 2004):
She could cut letters and words in the ice of the canal
with the blade of her skate,
like the long-ago Dutch poets.
Who were these Dutch poets? Did they really cut poems in the ice? Vondel and Bredero wrote poems about skating during the sixteenth and seventeenth centuries (known as Europe's Little Ice Age), and some of Vondel's poems are even short enough to skate (two words: U / Nu!). The whole thing is probably apocryphal, but it's a lovely image nonetheless.
[The painting is by Hendrick Avercamp, A Scene on the Ice (1625). You might recognize it from Night at the Museum: Battle of the Smithsonian (actually, it belongs to the National Gallery, and you can skate there, too).]