About this passageRabbi Tarfon taught after the destruction of the Second Temple, when the work of rebuilding a way of life was plainly larger than any lifetime. The two sayings are usually read together because the second answers the panic the first produces: the task really is too big, and finishing it was never what was being asked. In the original the work is the study of Torah; the formulation has since been applied far past it.
"The day is short, the task is great, the laborers are sluggish, the reward is much, and the Master of the house is urgent."
He also used to say, "It is not thy duty to complete the work, but neither art thou free to desist from it."
In plainer words
The day is short and there is far more to do than can be done, and the person who set the work is not relaxed about it. And also: finishing it is not your job. Neither are you allowed to walk away from it.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.