The day is short and the work is great. Rabbi Tarfon is comparing the obligation we have throughout our lifetime to master Torah and understand Torah. We were created for this reason: to grow and to achieve perfection, which happens through learning Torah. He's comparing it to a worker who has a very big task ahead of him and has to complete it in one day. A person's life may be 70 years, maybe 80 years. But when we consider the life we have and the vastness of the Torah, as the Pasuk (verse) in Iyov tells us, “The Torah is longer than the earth and wider than the sea.” That's the Torah we're dealing with. Every ounce of Torah we learn is just a drop in the bucket, and there's so much Torah out there. So the 70, 80 years that we have represent a relatively short amount of time for the amount of Torah we have to learn. That's what we mean by the day being short and the work being great. We have a life, that's one life, and we have so much work to do. We were created for this reason. “Adam le'amal yulad,” man, was created to toil and to learn Torah and to grow. So we have to recognize that there's so much Torah out there; we have to do as much as we can.
But the problem is, man, by nature, is lazy. People don't naturally want to pursue mastery in Torah. They'd rather relax. They'd rather chill. They'd rather just have a good time and not have to work hard at learning Torah. By recognizing that we sometimes have this trait of laziness, we can work on ourselves, conquer it, and become people who work hard at Torah. But we have to know as well that the reward is great. If you study Torah, both in Olam HaZeh (this world) and in Olam HaBa (the World to Come), Hashem is going to take care of you, and He's going to give you a tremendous reward.
But the master of the house is insistent that we learn. We have the Pasuk that tells us from Yehoshua, “You have to learn Torah day and night.” Hashem is telling us that studying Torah every available moment we have is a necessity, for our own benefit, though—for the reward. If we want to work hard in this world, learn Torah, and become Talmidei Chachamim (Torah scholars), the reward for us at the end of days and in Olam HaZeh will be great. And, lo aleinu (God forbid), the inverse: for somebody who doesn't study Torah, if they come to this world and get lazy and don't really push themselves to grow, then they're wasting their time. They're wasting their time in Olam HaZeh, and the punishment for that type of person is there as well. But we only want to focus on the good things, the sachar (reward) for a person who does study Torah in Olam HaZeh in this world and in Olam HaBa as well.