What happens in the sky
An ingress is the moment a planet crosses from one zodiac sign into the next. At this event, Neptune leaves its previous sign and enters Capricorn. The zodiac is measured along the ecliptic — the apparent path of the Sun through the sky — and divided into twelve 30° segments. When a planet's ecliptic longitude crosses a 30° boundary, an ingress occurs.
Neptune — the body
In the classical tradition, Neptune carries dissolution, imagination, and what resists clear outline. Its ingress marks a change of tone for these themes: the background colour of the sky, so to speak, shifts.
Capricorn — the ground it enters
Capricorn is characterised as structured, enduring, and oriented toward the long arc. When Neptune travels through Capricorn, its familiar themes take on that flavour until the next ingress.
How long it lasts
The duration depends on the body. The Moon passes through a sign in roughly 2.5 days; the Sun and inner planets in a few weeks; Jupiter takes about a year; Saturn roughly 2.5 years; the outer planets much longer still.