This is a way to describe luck in Navajo. It means “You’re lucky” if used in this way.
T’áá íyisíí (áyisíí) nízhánee’ is a way of saying ‘You are a really lucky person!’ – in other words, when something very good happens to someone.
There are also the words shízhánee’ (I am lucky), bizhánee’ (he/she/it is lucky), and nihízhánee’ (we, or you all, are lucky).
In the traditional context, how fortunate one is is affected by their conduct in life, and is less about pure luck with no ability to affect the outcome.
One particular usage of this word is Shizhané’é, which is a round dance commonly performed during an Enemy Way ceremony. The ceremony itself is said to purify one of the evil one obtains in warfare. The Navajo tale of the warrior twins tells of Monster Slayer’s return from his journey of conquering the giant monsters. He began to suffer from psychological changes from fighting, so he underwent the ceremony.
In this way, you can begin to understand that Navajo luck leaves a lot of room for us, as humans, to affect the outcome of things we do not fully control.