A netlist is the plain text form of a circuit: one line per part with the nets it connects to, then the simulation commands. LTspice makes one every time it simulates, but only on a computer where LTspice is installed, and it hides the file next to the schematic. People need it for many reasons: to run the circuit in ngspice on Linux or in a Python script, to paste a circuit into a forum question or a report as text, to compare two versions of a design line by line, to check a homework schematic for a missing connection, or to move a circuit into another tool such as KiCad's simulator.
This page builds the netlist from the drawing the way LTspice does. Wires are joined at their ends and wherever a wire end, pin or label lands on another wire. Labels with the same name are one net, and the label 0 is ground. Labelled nets keep their names, and the rest are numbered N001, N002 and so on in the order the parts are listed. Pins that touch nothing get their own NC names so the simulator warns you, instead of being silently joined.
Each part line follows LTspice's rules: the reference (with the type letter added when the name does not start with it, so an op-amp U1 becomes XU1), the nodes in the symbol's SPICE order, then the value and any extra parameters such as tol=1% or AC 1. Transistors get their substrate node like LTspice writes it, and parts left on their default model get the matching .model line (for example .model D D). The micro sign µ is written as u, which every SPICE reads. All .tran, .ac, .dc, .step, .param, .meas and .include lines come after the parts, ending with .backanno and .end.
We checked the connections against netlists that LTspice itself wrote for the same schematics: every part landed on the same nets. Library parts with their own symbol, such as a regulator chip, need their .asy file to know the pin order; without it they are listed as a comment instead of a wrong line, and the page names the files to add.