Encounter
From Kerbal Space Program Wiki
An encounter occurs when the projected flightpath of a spacecraft intersects the sphere of influence, representing the gravitational field of a celestial body. Encounters are displayed in map view by an encounter node and a colorized trajectory path.
Upon arrival at an encounter node, the game switches the orbital reference of the craft to that of the encountered celestial body; an ideal moment to plan maneuvers to stabilize into an orbit. Orbits can be disturbed when time-warping across sphere of influence changes. To maintain a carefully planned orbit, the time warp should be slowed down to 1× as the ship approach the sphere of influence change.
Encounters are essential for all Munar and interplanetary travels. And to get back home.