The lineweight is the thickness is the line. For example a regular Ballpoint pen with nominal lineweight if 0.5 is should draw a line with thickness of 0.5 mm.
Similarly in CAD design we set the thickenss that the line are going to be printed on paper. You should not use more that four lineweights (line thicknesses) in drawing. Each lineweight should have noticable thickness difference from the other 3 lineweight, otherwise it will be hard to tell the difference between lines with closeby thickness
0.15 mm: Axis, Hatching
0.30 mm: General use, notes, dimension, elevations etc.
0.60 mm: Section Lines, Columns sections
0.80 mm: Rebars
0.30 mm: Rebars on sections