A day's journey is a way of measuring how far someone can travel in one day. It's like when you go on a long car trip with your family and your parents say "we'll drive for 4 hours and then stop for lunch." That 4 hours of driving is like a day's journey.
But instead of measuring in hours, people used to measure distances in how far a person or animal could walk in a day. This measurement was different depending on where you were in the world, because some places had better roads or easier terrain to walk on.
Overall, a day's journey was just a way for people to know how far they could go in a day, without getting too tired or stopping to rest. It helped them plan their travels and make sure they arrived at their destination before it got dark or they ran out of food and water.