The yellow circles show you the orbital path of stuff - that's why you can usually see a whole bunch of yellow lines across your screen when you turn away from a star after arriving in a system - it's the orbital paths of all the planets (and other stars, if applicable) in the system. The blue circles seem to resemble the "tunnel" in Frontier that you flew down toward your target, only in this case they seem to emanate from a planet rather than the orbiting space station that is my actual target. I'm guessing it might be the "thing of strongest gravitational pull in the direction you're currently heading" but I don't actually know for sure.
I get the "slow down" thing too, sometimes, even when I've been managing my speed (putting it right in the middle of the blue section as I start getting close to my destination) as normal like I always do when approaching a space station. It's more of a minor annoyance than anything else but it'd be nice to know what causes it. Sometimes when I'm getting close to my destnation and am lining myself up between the planet and the space station (so I'm facing as near as possible to the station entrance when I drop into normal space), I'll see an NPC rectangle zoom past me and alter course to fly back again so I'll deliberately speed up so as to avoid an interdiction - because I'm really close to my destination by that point I usually haven't built up enough speed to overshoot before I get the "Safe to disengage" message (or whatever it says, I've seen it a million times and I can't remember it off the top of my head

), but it has happened once or twice. Still, I'd rather have to turn around and reposition myself than get interdicted and have to run away from that
and then turn around and reposition myself

.