It is known as such because Shema is the first Hebrew word in the verse. Jews sing it every morning in prayer across the world. The definition of echad can be found in Strong's Hebrew and Chaldee Dictionary, Strong's number 259. It is from a root of Strong's number 258, achad, which means to unify, or to go one way or another, a unity. Context determines meaning.
Lets look at two passages in Genesis.
Lets compare Duet. I just stated that YHWH was supposedly a singular being . The reason I say that is there are numerous examples in the Hebrew texts, which find more than one YHWH in the passage. Here are two examples. John was simply relaying a very Jewish idea that the Messiah was to be G-d the Word.