Answer by taking a quote from the following article:

Namdev, also transliterated as Namdeo and Namadeva, (traditionally, c. 1270 - c. 1350) was a poet-saint from Maharashtra, India who is significant to the Varkari sect of Hinduism. He is also venerated in Sikhism, as well as Hindu warrior-ascetic traditions such as the Dadupanthis and the Niranjani Sampraday that emerged in north India during the Islamic rule. The details of Namdev's life are unclear. He is the subject of many miracle-filled hagiographies composed centuries after he died.

Namdev's padas are not mere poems, according to Callewaert and Lath. Like other Bhakti movement sants, Namdev composed bhajans, that is songs meant to be sung to music. A Bhajan literally means "a thing enjoyed or shared". Namdev's songs were composed to be melodious and carry a spiritual message. They built on one among the many ancient Indian traditions for making music and singing. Namdev's bhajans, note Callewaert and Lath, deployed particular species of Raag, used Bhanita (or Chhap, a stamp of the composer's name inside the poem, in his case Nama), applied a Tek (or dhruva, repeated refrain) and a meter than helps harmonise the wording with the musical instrument, all according to Sangita manuals refined from the 8th to 13th centuries.  The musical genre of Namdev's literary works was a form of Prabandha - itself a very large and rich genre that includes dhrupad, thumri, tappa, geet, bhajan and other species. In some species of Indian music, it is the music that dominates while words and their meaning are secondary. In contrast, in Namdev's bhajan the spiritual message in the words has a central role, and the structure resonates with the singing and music. The songs and music that went with Namdev's works, were usually transmitted verbally across generations, in a guru-sisya-parampara (teacher-student tradition), within singing gharanas (family-like musical units).  Callewaert and Lath state that, "each single song of Namdev is a musical and textual unit and this unit is the basis for textual considerations". The unit contained Antaras, which are the smallest independent unit within that can be shifted around, dropped or added, without affecting the harmony or meaning, when a bhajan is being sung with music. In Namdev's songs, the dominant pattern is Caturasra, or an avarta with the 4x4 square pattern of musical matras (beat).

How do they relate to Namdev?
Namdev composed bhajans,