Instagram announced “will soon be ordered to show the moments they believe you will care about the most”. Instagram used to organize the feeds in reverse-chronological orders (i.e. they appeared in your feed when they were shared). Instagram says they are doing this to improve user experience and they are doing this because users might be missing photos and videos that would matter to them.
The order of photos and videos in your feed will be decided on if you will be interested in the content, relation with the person posting it, and timeliness. In the initial phase, Instagram is just going to be optimizing the order, which means you will still be seeing all the posts.
So worry not, if your best friend posted a picture, or you missed a video from your favorite musician. The content that means to you will be waiting for you! This change will start coming in the coming months! So stay tuned to get a Personalized feed in Instagram.