the ground node (node 0)
Every voltage is really a difference between two points, just as saying a hill is 30 metres high only means something relative to sea level. SPICE needs one agreed reference point that everything is measured against, and that point is the ground node, always named node 0. Its voltage is defined as exactly 0 volts.
When SPICE reports 3 volts at the output, it means 3 volts above node 0. Node 0 is special: it is the one node whose voltage you do not solve for, because you have declared it the zero. Every other node's voltage is found relative to it. In a netlist you must connect your source's return and your circuit's common rail to node 0 (in LTspice you place the ground symbol). Without it, the simulator has no anchor and the equations have no unique solution.
The most common SPICE error message a beginner meets is This circuit has no ground or Node X is floating. Every circuit must have a node 0, and every part of the circuit must trace a path of components back to it, or there is a section whose voltage is undefined. Note that SPICE's node 0 is a mathematical reference, not necessarily the earth ground of a wall outlet; that distinction matters for safety but not for the math.
Mark the bottom rail of a divider as node 0, set the top to 9 V, and SPICE returns out = 3 V meaning 3 V relative to that rail. Re-define a different node as 0 and every reported number shifts, even though the circuit behaves identically.
Node 0 is the agreed zero; all voltages are quoted relative to it.
Forgetting to place a ground is the number-one reason a simple simulation refuses to run. SPICE's node 0 is a reference for the math and is not automatically the same as mains earth.