The Wilmot Proviso showed that the North held essentially to a free soil attitude with regard to the expansion of slavery. This means that the North did not want slavery to expand out of the area in which it was already in existence.
The Wilmot Proviso stipulated that no land taken in the war with Mexico could become slave territory. By saying this, the Northern members of Congress who supported the proviso were trying to ensure that there would be no spread of slavery. They were, thereby, showing that they held a free soil attitude towards slavery.