system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: According to data we have discovered 14% of all organisms on earth. Where does this number come from, if the other 86% of haven't been discovered yet (and therefore we don't know if they exist)?
gpt
Statistics like this are created based on looking at what is identified within a group.

Perhaps an easier example.

Let's say people are inspecting defects in a product.  Someone in charge intentionally adds 10 defects.  Then they watch and see what comes through the line, what is discovered by the process.  If people only find 3 of the defects, then they can estimate they're catching 30% of the defects overall, letting 70% of the defects go through.  On the other hand, if all 10 defects are discovered, then they know they're catching all or nearly all of the defects.  The percentage of things they know about should roughly match the percentage of things they don't know about.

It applies to other statistics as well, like crime stats.  They can look at crimes they know happened but weren't reported through official channels, and look at crimes they know about and were reported. Looking at the difference shows about how many crimes go unreported.  It is not exact, but if people are careful about how they create the stats they can be fairly accurate. 

For counting species there are several ways it can be done.  One way is like above, to have one group track the number of species in an area and another group figure out how many are new. Another method is a linear regression, figuring out an approximately how many species there should be based on estimates and comparing it to how many have actually been identified.

Also, most of the species that aren't discovered are small things.  We're down to small numbers of new birds and mammals, often they are sub-species that get reclassified as a new species, or they're highly specialized species living in a remote and small geographic area.  

It is mostly bugs, fungi, and other small organism that are being discovered in large numbers. These are things that are hard to spot and identify, many only identified because of genetic testing on tiny or microscopic organisms.