a general position
A general position is a point placed 'in general' — deliberately NOT lying on any rotation axis, mirror, glide, screw or inversion centre. Because it hides on no symmetry element, nothing but the do-nothing identity leaves it fixed, so its site symmetry is 1 and its multiplicity is the largest any site can have in that space group.
Apply every symmetry operation of the space group to one general point and each operation lands a distinct copy somewhere in the cell. The count of these copies is the space group's general multiplicity, and their coordinates — written as triplets like (x, y, z), (-x, -y, -z), and so on — are the 'general equivalent positions' listed at the very top of that space group's page in the International Tables. In P2_1/c the four are (x, y, z), (-x, -y, -z), (-x, 1/2+y, 1/2-z), (x, 1/2-y, 1/2+z).
That list of general positions is more than a table of dots: read as coordinate transformations it IS the complete set of the space group's symmetry operations, spelled out in a form software can apply directly. Atoms in low-symmetry crystals — most organic molecules, for instance — almost always occupy general positions, because a molecule of no particular symmetry has no reason to line up with a special axis or plane.
In triclinic P-1 the only symmetry beyond translation is an inversion centre, so the general position has just two images: (x, y, z) and (-x, -y, -z). Every atom of a molecule in P-1 comes with an inversion-related partner across the cell.
The general-position coordinate list is the space group's symmetry operations written out.
The number and coordinates of the general positions belong to the space group alone, not to any particular crystal — but which atoms actually sit on them is a fact about the specific material you are studying.