Lineweight is the visual thickness of lines. The use of lineweight is critically important to maintaining good legibility and professional appearance in drawings. See this post for general information on lineweight.
In AutoCAD, lineweight is an object property, meaning that every object in an AutoCAD drawing can be assigned its own lineweight. By default, every object is assigned the ByLayer lineweight, meaning that the object assumes the lineweight assigned to its layer. Like any object property, this can be overridden on an object-by-object basis.
It’s considered good practice to use layers to organize object lineweight, rather than setting object lineweight on a per-object basis. In other words, unless there is a compelling need to do otherwise, always keep AutoCAD object lineweight set to ByLayer.
TO CHANGE LINEWEIGHT OR LINETYPE ON A BYLAYER BASIS:
1. Load a linetype into your drawing. (See above.)
2. Type LAYER to bring up the Layer Properties Manager palette.
3. Click on the layer whose properties you wish to change.
4. Choose the desired linetype or lineweight. All objects in this layer with the BYLAYER property set for linetype or lineweight will inherit these settings.