Your verb is stand. The auxiliary verb is to be. The present tense of your verb phrase is: "are standing".
Were standing is called the past progressive tense. It is describing an on-going activity in the past that ended before the present moment.
It is also a play on a famous sentence written by Isaac Newton, who credited the work of others before him, as one who discovers by building on previous discoveries:
"If I have seen further, it is by standing on the shoulders of giants."
The first person to have actually written these words was Bernard of Chartres in 1159. It was probably inspired by Greek mythology, where the blind giant Orion carried his smaller servant Cedalion on his shoulders to provide himself with 'vision'.