Very simple 2D drawing software

I need to print a series of parallel lines that are precisely spaced at 10.0 mm to each other. I will use the printed piece of paper to stick to a wooden piece and then cut it, using the paper as a guide.

I don't need fancy things like CAD. Something simple and it would be nice to be free.

I don't really know how to (what keywords to use) search for something like this.

What you describe could be considered parametric design software. There are a few programs that will do as you require, but you will likely discover that they also have a 3D component. Even the simpler ones such as SketchUp will allow for 3D use, but you don't have to use it. Keep all your sketching in one plane and you'll be all set.

Consider to perform a search on YouTube for SketchUp tutorial to see how it works.

I'm a fan of a parametric drawing program known as SolveSpace. It also is free and has a few tutorial videos on the YouTube.

Well, shucks and by golly, I almost forgot one of my more commonly used programs, strictly 2D drawing, known as Inkscape.

With Inkscape, you can position the guides at 10 mm intervals and use them to position your lines. Alternatively, you can use the slightly more complex method of creating clones and spacing the clones at 10 mm intervals. Either way, it won't get simpler than that. Go into preferences, change the units to mm and cast away!

You can also simply use LibreOffice tools like Draw, Write or even Calc (of course you can also do this with Microsoft Office if you already have it installed in your computer). They can insert simple diagrams, you can select the measurement system you want (pixels, centimetres, etc). Just make sure to select actual size when printing the file and there you go. Place the paper on top of the wood and it will have the actual sizes of what you have drawn.

https://www.libreoffice.org/

Try https://www.sketchandcalc.com/area-calculator. You can design 2 lines, or may be a rectangle and can specify their distance. It's fairly simple.

Though it's purpose is different but it can help in your case.