If you have a website/Blog/Channel or anything that is live on the web,then Guest posting is crucial for you,and you need it more than you know.
Guest Posting is simply posting to websites that don't take money for you posting on their website,like they leave this white space for you to write whatever you want,and most probably all what they'll do need is for you to create an account.\
So what do you do with this white space provided?
You for sure make a free advertisement for yourself, write your motto,what you do and what you could help with,your website's link/Youtube's Link/Social Media Links and all the links that you have.
Now what you are expecting is that people will go to this website and see your post, right?
No, this isn't exactly what happens, but when someone searches for something that you have on your website, Google finds that you're a suitable reason because your link is found on many other websites so that means that you are worthy of appearing hopefully at the top of the list.
When your website's link is found on another website, that is called a backlink, and you better create it yourself and don't buy it, because nowadays Google Identifies it and they could really harm your service in very bad ways by either stopping their ads or not showing you in the search results.
So I hope that you've got the answer you wanted and now just search for Guest posting websites and make the backlinks yourself (I'm sorry I didn't provide links but I fear that websites change all the time and I don't want to give you broken links)