Check the picture below.
the piecewise function, has two subfunctions or behaviours, one if x < 1, meaning less, not equals, but less than 1, so it doesn't include one, so it has a "hole" on that endpoint.
and the second behaviour of the piecewise is if x ⩾ 1, larger or equals than 1, so it includes 1, so it has a solid ball on that endpoint.
the piecewise function, has two subfunctions or behaviours, one if x < 1, meaning less, not equals, but less than 1, so it doesn't include one, so it has a "hole" on that endpoint.
and the second behaviour of the piecewise is if x ⩾ 1, larger or equals than 1, so it includes 1, so it has a solid ball on that endpoint.