Lineweights help differentiate the purpose of one line from another. Lineweights determine how thick or thin entities appear both on the screen and when printed.
By default, each drawing has three main lineweights : DEFAULT, BYLAYER, BYBLOCK, and many additional lineweights in millimeters (or you can use inches).
When you create an entity, it is created using the current lineweight. By default, the current lineweight for a new entity is BYLAYER. This means that the entity lineweight is determined by the current layer. When you assign BYLAYER, changing a layer's lineweight changes the lineweight of all the entities assigned that layer (if they were created using the lineweight BYLAYER).
You can also select a specific lineweight (or DEFAULT) as the current lineweight, which overrides the layer's lineweight setting. Entities are then created using that lineweight (or the DEFAULT lineweight), and changing the layer lineweight has no effect on them.
As a third option, you can use the lineweight BYBLOCK property, in which case new entities are drawn using the DEFAULT lineweight until you group them into a block.
The entities then inherit the block's lineweight setting when you insert the block into a drawing.
If you choose a lineweight that is less than .025 millimeter, it displays as one pixel when you create your drawing. When you print your drawing, it prints at the thinnest lineweight that is available for your printer.
You cannot assign lineweights to planes, points, TrueType fonts, or raster images.
LWEIGHT: Sets the current lineweight, lineweight display options, and lineweight units
PLINE: Creates two-dimensional polylines
System Variables Reference