This world is like a lobby in front of the world to come. The world that we're in is like a hallway, like an entranceway to the main stage, to the really important part of existence. Rabbi Yaakov is telling us that the world that we're in physically is not an end in itself, but it's all a way to enable us to get to the ultimate destination, which is Olam Haba (the world to come). The world that we're in is a means to an end. It's basically meaningless unless we're focused on what's really valuable, which is a person going to Olam Haba, the world that we're currently in.
A person has the ability to lose himself in the pleasures, enjoyments, and distractions that he's in. But really, that's all futile in the face of Olam Haba, which is the ultimate pleasure, the real type of pleasure. It's also an enjoyment that a person experiences that's everlasting, that's forever, that's not bound by time. A person in Olam Haba could enjoy something, but that enjoyment is fleeting. It's also qualitatively a very low type of enjoyment, because it's just a person fulfilling a biological need. So comparing that with the spiritual enjoyment of Olam Haba, it's really not comparable.
Hateken atzmecha ba-prozdor, kedei shetikanes la-traklin. Consider yourself in the lobby so that you have the ability to enter the palace. A person should spend his time the right way. When a person is going into the palace to meet the king, he'll spend the moments before he enters brushing his hair, putting on the right clothing, and making sure that he's presentable to go in and eventually do proper things in the palace. It's the same with this world. A person should spend his time in this world, using it to be zocheh, to merit, to enter the world to come, by doing teshuvah (repentance), by living. We learn in Torah, and by doing ma'asim tovim (good deeds). By doing that, a person will eventually enter Olam Haba. So he has to look at this world as an opportunity to work on himself, so that he can enter the next world, chayei Olam Haba (life of the World to Come).