How important is SEO?

I'm fairly new to online business and websites overall and I've heard so much about SEO. Currently visits to my site have gone down drastically. I don't even know how I got my first visitors tbh but after that first one I had a short run of visitors and customers come to my website. Now, I get one every now and then :( tbf I haven't advertised anywhere yet...anyway my point is, if I focused more on SEO and tweaking areas of my website, will that help increase visitors and in turn sales? What should I look at specifically? What will be most helpful in my case?