"In my youth, I was considered a bright and diligent student. How did I accomplish this? I tricked my yetzer hara. Other children had great plans at the beginning of the school year for the whole year, but in the end they failed. I said to myself, "I am going to plan just for today - and set goal for this day only". The Satan, not being interested in a single day, left me alone. The next day, I again just made plans for that day, and so on until the end of the year." - Klausenberger Rebbe
This is a tremendous yesod. How many of us are able to just take things one day at a time? We dwell in the past ("Oh, man, why'd I do that?!), and in the future ("What if this happens?"). The pasuk is telling us to stop dwelling on the past or worrying about the future. Right now we are standing right here. What are we going to do with this moment?