用一杯水去救一车着了火的柴草:use a cup of water to put out a burning cart of firewood. it means that sth is far from enough to solve a problem，indicates there is no use，in vain.
杯：cup, here is a measure word ( a cup of)
水：water, in the legendary story related, actually it refers to a cup of tea.
车：cart，here it is a measure word means (a cart of)
薪：in old days, it refers to faggot or firewood that could be used as fuel, nowadays 薪 means salary or wage（薪水 in Chinese） .
用......去......: use...to do...