After high school, everything changes-- not to sound too cliché. The biggest thing that changes is your best friends aren't with you every day at school anymore. Unless your best friend has the exact same major at the exact same school with the exact same schedule, you won't be together every day like you used to be. Some friends move away for college, some friends take a lot of hours a semester, and some friends have to work and go to school. All of this causes distance that wasn't ever there before. Having that distance is hard and a lot of friendships won't be as strong as they used to be and some won't even make it-- except the good ones.
Distant best friends are the ones that you don't have to talk to everyday to know that y'all are still close, or you might not even have to talk for months, but when you pick up the phone and call them it's like nothing has changed. You both find some time in your busy schedules to text or chat for just a few minutes. You are still best friends, you still love each other, and you still care-- now you all are just a lot busier.
My distant best friend has a job at a hospital and works a ton of hours, goes to school, has a boyfriend, and has to balance all of that with family/friend time. I have three jobs, I go to school, I volunteer, and I also have to balance family/friend time. Neither one of us has a lot of free time, but when we do, we are checking in on each other. Sometimes if our schedules match, we may meet for breakfast or lunch and a chat.
No matter how long it has been since we've talked, no matter how much we have tried to hang out but can't, or whatever else happens she will always be my best friend and I will always love her. College is just hard. Working is hard. Learning how to balance everything is hard. In college everyone starts to go on their own path, and sometimes those paths don't cross often. You have to learn who you actually want to put effort towards in a friendship during college and whose friendship you want to leave in high school. You learn that lesson fast and you learn it hard.
Once you learn who you want keep in your life and who you don't, you learn to appreciate the distant best friend relationship. Because we're not in high school anymore, life is real now.