Ovulation Testing....Please explain....

Hi ladies

After visiting my OBGYN, I explained to her that my fiance and I will start TTC this summer. She recommended I started taking ovulation strips/tests. I didn't think of asking her at that moment how these work. She recommended purchasing the tests on Amazon. After some researching, I think I found the ones I want to buy but I need some clarity on this so I know what to do....

I noticed some of the women posted pictures of their tests and had them organized in order of when they took them. This might sound like a silly question but can someone explain this to me since I'm very new to this. Thanks I really appreciate it!