What a circle is
Casting a circle means marking out the space you are working in and treating everything inside it as set apart until you finish. In most published Wiccan ritual the circle is walked or traced clockwise, often starting in the east or north depending on tradition.
Why people cast one
Three readings turn up constantly, and many practitioners hold more than one. Protective: a boundary against unwanted influence. Focusing: a container that keeps attention and effort in one place. Symbolic: a stage marking, which says that ordinary time has paused. Wicca inherited the practice largely from ceremonial magic, where the protective reading was dominant. Modern Wiccans use all three interpretations.
A circle is not a ring of salt
Salt lines are one option among many, and a poor one indoors with pets, outdoors on soil, or on any floor you would rather not scrub. The circle is defined by intention and marking, not by materials.
Ways to mark it
Point with a finger, an athame or a wand as you turn. Walk the boundary. Trace it with your eyes from a seated position. Lay out a cord, some stones, tealights or LED lights. Say the boundary aloud. Draw it once on a cloth and use that cloth every time.
If walking round the room isn't available
It very often isn't, and the tradition is not fussy about this. Turn where you sit. Trace the circle in the air in front of you. Trace it on your palm. Cast it entirely in words. A circle cast from a chair or a bed is a circle.
A simple version
Ground and centre. Face whichever direction you are starting from. Turning clockwise, or tracing clockwise with a hand or your eyes, say something plain: 'This is my circle. What happens here is deliberate.' Do the thing you came to do. Then reverse it, anticlockwise, and say the circle is open. Plain wording works perfectly well.
Closing properly
Always open the circle again at the end, and do it in the reverse direction to how you cast it. Thank anything you called. Then eat or drink something and put the room back to ordinary. Skipping the closing is the most common beginner habit, and it is what makes a ritual feel like it never quite finished.