We use a special formula to determine which photos make it to the Explore tab, and we do our best to make sure everyone has a fair chance of seeing their photos there.
The goal of the formula is to surface the most recently interesting photos based on a variety of factors. The formula doesn't simply count likes; it takes into account much more than that to make sure new people with fewer followers also have the chance of being discovered. We're constantly tweaking the formula in hopes of surfacing interesting photos from all members of the Instagram community.
A notification will appear in your news feed when your photo makes it the Explore tab. You'll also get a message if you have notifications on for Instagram.