Plane angle is an angle between the intersection of two straight lines or the intersection of two planes.

Radian is the SI unit of plane angle and it is denoted as 'rad'.
Where 'radian' is the angle subtended at the centre of a circle by an arc whose length is equal to the radius of the circle. Because it has two dimensions.
π radian \(=\) \(180°\)
Then, \(1\) radian \(=\)  180°π.
Radians are used to measure angles.
Solid angle is an angle formed by three or more planes intersecting at a common point and also known as 'angle formed at the vertex of the cone’.

Steradian is an SI unit of solid angle and it is denoted as 'sr'.
Where 'steradian' is the solid angle at the centre of a sphere subtended by a portion whose surface area is equal to the square of its radius of the sphere. Because it has three dimensions.
Solid angle are used to measure the amount of the field of view from some particular point that a given object covers.
Luminous intensity is the perceived power per unit solid angle. If a lightbulb has a \(1\) lumen bulb and the optics of the lightbulb are set up to focus the light evenly into a \(1\) steradian beam, then the beam would have a luminous intensity of \(1\) candela.