New Mexico was a territory of the United States for over 60 years before being admitted to the Union as a state. The long delay came about for many reasons, some of them based on race, some based on politics. William Taft was president when the long struggle finally ended and New Mexico became a state. Taft signed the bill that allowed New Mexico to create a constitution and he signed the bill that accepted the constitution and made New Mexico a state. In these ways, he helped New Mexico enter the Union.
Taft also helped to get Congress to agree to let New Mexico in. New Mexico's statehood was a controversial topic and the political support of the president helped to push Congress towards accepting New Mexico.
So, Taft helped New Mexico become a state by pushing Congress to admit it and by signing the bills that actually made statehood happen.