A safespot is any saved location (aka bookmark) to which you can warp and not end up on the grid of a warpable celestial.
Some are better than others for various purposes.
You can be scanned down and warped to anywhere in a system if you're uncloaked, and Mobile Observatories can decloak you once your cloak timer ends.

• inline safes are convenient but insecure since they're easy to reproduce:
  1. save a bookmark mid-warp
• unaligned safes are a bit safer since they're harder to reproduce:
  1. make an inline safe then warp to somewhere that wasn't one of the two points you warped between to make it
  2. warp to the inline safe and drop a second inline safe (this second bookmark is now not aligned to any celestial)
• deepsafes are currently the safest. they're bookmarks which are more than dscan range (14.3 AU) away from any spot in a system which can be reached by normal means:
  • method 1: save Sansha Incursion
    1. if a Sansha Incursion is occurring in a constellation, you can save its beacons as bookmarks (the BMs will persist after the sites disappear)
       • they're often far away from all other celestials in a system & outside dscan range of everything
  • method 2: survive to downtime in Abyssal Deadspace
    1. enter Abyssal Deadspace within ~18 minutes of downtime (so any time between ~10:42:00 and ~10:59:55) and survive until the servers close
       • use a ship & fit which can either tank the filament's DPS until DT or which can kill all the rats in it
         • I use Tranquil Gamma filaments for this purpose since they make passive tanking easier; virtually all hulls can be made to omnitank a Tranquil Gamma's max of 75 DPS
    2. log in after DT and save the location if it's good
       • when you login after DT, the ship will be returned to the system in which it activated the filament at a random spot (placement seems random like with Incursion beacons)
       • WARNING! if you've made an Abyssal safe and traveled to another system to make more, do not activate a filament after the countdown reaches ~5 seconds
         • activate the filament well in advance of shutdown, or you can be teleported to the last system in which you made an Abyssal safe. this is an exploit and can get you banned
           • I'll clarify since this has come up a lot:
             • using this mechanic to make deepsafes is not an exploit; the exploit is changing systems via the mechanic, which is completely avoided by not doing it near server shutdown
               • I recommend a cutoff of ~5 seconds because actual server shutdown can vary between a few seconds before the counted 0 to up to 70 s after
                 • once you've crossed DT in the Abyss from a given system, you're no longer at risk of tunneling there on subsequent DTs (until you travel to another system)