Orbital elements are parameters that describe an
orbit (the path of an astronomical objectgravitationally bound
to another, particularly, a repeating pattern) according to a
reference plane and a direction through that plane.
Commonly used are six elements to describe a Keplerian orbit.
Two of them describe shape and size of the orbit:
semimajor axis - half the longest diameter of the ellipse.
The other four elements describe the orbit's attitude and the current
phase, the latter which can be thought of as the current position
of the orbiting body within its orbital path.
These four are more challenging to describe so I'll use the following
orbit-related terms:
plane of reference aka reference plane - an otherwise-defined plane fixed in space, within which the host body resides, used as a basis for describing the orbit; often used for the solar system is the ecliptic.
orbital plane - plane in which the orbit resides; note that an undisturbed Keplerian orbit is always planar.
line of nodes - the line at the intersection between the orbital plane and the plane of reference. This passes through the ascending node, the host body and the descending node.
reference direction - an otherwise-defined direction (ray) from the host body through (i.e., embedded within) the plane of reference, specifically that is not coincident with the line of nodes.
ascending node - the point on the line of nodes through which the orbiting body passes from the side of the plane of reference declared "down" to that declared "up". In the case of solar system orbits, the ascending node passing the side of the ecliptic to which Earth north points, is also known as the north node.)
descending node - similar point for the orbiting passing to the opposite "downward" (within the solar system, aka the south node).
line of apsides - line through the two foci of the orbit's ellipse, which is along the longest diameter of the ellipse. For a circular orbit, any specified line embedded within the orbital plane that is not coincident with the line of nodes will serve.
The remaining four orbital elements:
inclination - angle between the reference direction and the orbital plane.
argument of periapsis - angle within the orbital plane from the line of nodes' ray that passes through the ascending node, to the line of apsides' ray.
longitude of ascending node (LOAN) - angle within the reference plane from the reference direction to the ray starting at the host body along the line of nodes through the ascending node.
true anomaly - angle within the orbital plane from the line of apsides to the line from the host body to the orbiting body's current location.
These six orbital elements are sufficient to describe the particulars
of the orbit at a given moment, given a reference time (epoch),
the orbit's position, a given reference plane, and a direction
across the plane considered to be "upward".
These givens could be provided by a given coordinate axes within
a given frame of reference, a position vector to identify the
orbit's position (e.g., to one of its foci), and a unit vector to
identify a reference plane and the "upward" direction crossing it.