On the lower right hand side of any observation page, you'll see a heading section called "Annotations" alongside Projects, Observations Fields, and Tags.
A key advantage of annotations is that they bubble up from observations to the iNaturalist Species pages. For example, on the Spotted Tussock Moth species page, the seasonality graph has a 'Life Stage' tab. The graph in this tab is driven by annotations on observations and can be used to visualize that larvae emerge later than adults accounting for the 'two-humped' seasonality distribution. Similarly, if you click on 'View more' in the photos section, the ability to filter photos by just 'Life Stage' = 'Larva' to see what Spotted Tussock caterpillars look like is also driven by annotations.
The benefits to the species pages from annotations will only emerge if enough people take the time to annotate observations. You can add annotations one at a time from each observation page, but that can be tedious. We've extended the 'Identify' tool to make it easy to quickly add annotations to encourage more people to help contribute this data. Here's how to do it. First, click on 'Identify' from the main menu.
Now you can click on the first of the observations to open the modal. Tap on the 'Annotations' tab where you will be able to select and add annotations. The real efficiency of using the 'Identify' tool comes from using Keyboard Shortcuts explained in the menu. For example, the arrow keys to move to the next observation, 'la' will add an annotation of 'LifeStage=Adult' and 'll' will add an annotation of 'LifeStage=Larva'. You can find out more about the 'Identify' tool in this tutorial video
Thanks for helping to add annotations. If you annotate a species to the point that the Seasonality graphs on the species page look pretty robust and find interesting patterns in life stage, sex, or flowering phenology, one last step we encourage you to do is add this information to the relevant Wikipedia article for the species. We use Wikipedia content in the iNaturalist species pages as do many other sites, and adding to Wikipedia is a great way to help grow Wikipedia and strengthen links with iNaturalist. In our experience, robust sources of phenology data, such as the data you are helping iNaturalist collect via annotations, are hard to come by, and this information is often either missing from Wikipedia or not well referenced.
To edit Wikipedia, first navigate to the relevant article on Wikipedia from the About tab on the iNaturalist species page. We recommend you sign up/log in to Wikipedia, but you don't have to. If the article doesn't have existing text on the subject of seasonality as it relates to what you annotated (e.g. life stage, sex, or flowering phenology) or if the text lacks a reference, you can help. For example, here's what I added to the Wikipedia article on Spotted Tussock Moths:
The moth flies from May to July; the larvae remain from July to September. {{cite web|title=Lophocampa maculata - Spotted Tussock Moth - iNaturalist|url=http://www.inaturalist.org/taxa/67691-Lophocampa-maculata|accessdate=29 June 2017}}