Naam Japo: means to recite the name of God, in this case, Waheguru is used. But that is not all, Sikhism says that there are many names that can be said, for example, Ram, Allah, Paramatma, etc.
NaamJapo or Naam Japna - Is the rememberance of God by repeating and focussing the mind on His name.
NaamJapo refers to the meditation, vocal singing of Shabads or hymns, from the Sri Guru Granth Sahib or the chanting of the various Names of God, specially the chanting of the word Waheguru, which means Wonderful Lord.
Naam Simran therefore is to concentrate on attributes of Waheguru and realize his grandeur and mercy.