Instagram is going to start using an algorithm to try to show users the posts they are most likely to appreciate, rather than just use a chronological timeline as it has until now.
The chronological timeline is simple and easy to understand, but it has drawbacks - if I don't look at Instagram within an hour (or a few minutes) of one of my friends' updates I'm unlikely to see it. An algorithmic timeline could be better - this is what Facebook uses, and it puts important news like engagements, and new babies at the top of my newsfeed.
As services like Instagram, Twitter and Facebook get bigger people follow more users, so there is more to see, but also more clutter.
The drawback of an algorithm is that can make some posts invisible - essentially if Instagram thinks that I'm not likely to like a person's post, it's likely to bury it. It's a black box in that it's not clear how the algorithm is worked out.
(I remember a story of someone getting so fed up that their posts about the unrest in Ferguson weren't getting seen on Facebook that she changed her status to 'married' so that it would appear in friends' feeds)
Some Instagram users are getting panicked that their friends won't see their posts and have been asking friends to turn on notifications so that friends will get an alert every time they post. This clearly isn't workable - imagine if you turned this on for 20 people - but it shows that people will try to game the new algorithm, just as they've been trying to game the current system by using lots of hashtags, tagging lots of friends in their pictures, even when they're not there, and messaging friends to ask them to like pictures. In my experience the most popular accounts, beyond the celebrity ones, are ones that are very specific to a genre - cats, landscapes, desserts - you get the idea.
What I suspect will happen once we have the new algorithm is that 'the rich will get richer' - if you currently have a lot of followers and get a lot of likes, you will start to get even more. Instagram is doing this both to make it a better experience for users, but also to make the platform a more engaging place for casual users, so it will show you more of what you have already said you like. This way you will return more often.
(Instagram already uses algorithms of course, in the 'search' tab, to show you popular pictures like the ones on the sites you already follow, and in recommending new accounts for you to follow, based on who your friends follow and sites that are similar to the ones you follow or have visited).
So what is likely to be included in the Instagram algorithm, to choose what pictures to show you?
The strength of connection to the account, including on Facebook - so you'll get to see more pictures from people you know, and even more from people you know well.
The frequency of linking pictures - if you often like Nigella Lawson's pictures you'll be shown all the news ones
The frequency of liking a type of picture - if you like pictures with the hashtag #dessert you'll be shown more #dessert pictures from accounts that you follow
The location - if you've posted a picture somewhere, you'll probably be shown friends' pictures from the same area.
The frequency of posting - Instagram will want to reward the people who put the work in - and besides their pictures are likely to be better.
At the same time I think it's likely that Instagram will start to penalise the spam tactics that people currently use to game it that I mentioned above - too many hashtags, too many users tagged - in the way that Facebook did in penalising the brand posts asking questions ("It's Friday - what's everyone doing tonight?) and other ways of fishing for interaction.
Instagram has always been gamed to a degree, and now that there's a 'black box' algorithm the gaming will increase - companies will offer to make brands' instagram pictures more popular - but essentially if you post good pictures, often, then you should be fine.