What is the Nishmas prayer?

Nishmas Kol Chai is a prayer of profound gratitude to Hashem, said at the end of Pesukei D'Zimra on Shabbos and Yom Tov and at the Pesach Seder.

Why do people say Nishmas for 40 days?

Saying the full Nishmas daily for forty consecutive days is a well-known segula when davening for a specific yeshuah, such as a shidduch or refuah.

Is a Nishmas card helpful for the 40-day segula?

Yes. A dedicated Nishmas card keeps the full tefillah beautiful and within reach so you can say it with kavanah each of the forty days.