Guide Line entities
Guide Line entities are infinite dashed lines used as guides to draw precisely. Guide lines do not interfere with regular geometry. Guide lines can also be hidden and erased independent of regular geometry. Draw guide lines with the Tape Measure tool.
You can use the Move, Rotate, and Erase tools to reorient a guide line. But, you cannot resize a guide line because guide lines are infinite in length.
Hiding/erasing all guide lines
Guide lines are usually created as a temporary means to build a portion of your model. Keeping too many guide lines in your model can decrease SketchUp's inference accuracy and display performance, so you might want to hide guide lines as you work or delete all guide lines at once when you have finished your model.
Use Edit > Hide to hide the currently selected guide line. Use Edit > Erase Guides to erase all guides in the current context.