Believe it or not these lines are all straight, go on get a piece of paper against them... The smaller you make the squares the more messy your perception gets.
What: This one should hit you fairly quickly, but if it hasn’t yet, it’s that the lines are all horizontal here, but appear to your brain to have angles… the illusion has a couple of names, it was first named the Kindergarten pattern in 1898  After a weaving pattern, and later as the Cafe Wall Illusion brought back into the light by Richard Gregory , which it is most commonly named now.
How To: You can play around a lot here with the customization tools, and there are some good things to note, if you reduce the contrast between the two colors the illusion will more or less disappear right in front of your eyes. You can also edit the sizes of the squares and find where the illusion will hit you the strongest. For me, it’s when the squares are rather small and a little tall. You’ll also be able to remove the line color altogether and again see the illusion evaporate.
Explain it: All in all, this is not very well understood, and from my research, there is no clear answer that doesn’t completely explode my smooth brain, it seems to be falling between the work that your eyes do to understand shapes and lines, and collides with what your brain does to piece together that same information.
I've researched these illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via email@example.com, be kind!