EVA will take notes for you in various ways.

System Notes

If you want to store a quick note for a particular star system:

1. Go to the Nav tab and find the system you're interested in. 
2. Use the map marker button to automatically get the system you're currently in, or
3. Type part of a system name in and search, then double-tap on the system name in the list of results.
4. Type your notes into the Notes box.

EVA will read these notes back to you each time you re-enter this system, if this option isn't switched off in the settings. They will also be sent to EDSM if you have given EVA your EDSM API key in settings.