A one way backlink is a link from one website to another where the site being linked to does not reciprocate
Take sites A and B for example, if A links to B, but B doesn't link to A, then B gets a one-way backlink from A.
This is called also a quality backlinking strategy, because it's best to receive a backlink from a website, but you do NOT link back to it. A -> B (A points to B) B <-> A(B do NOT links to A) BTW, if you want to exchange links with B, you should use a different website: A -> B C -> A (where C is another website that you can offer to exchange links with A) ... But do NOT link B with C for max results!
In one way backlink method a link come on website from another website. Like free directory submission one way backlink method and reciprocal submission two way backlink method.
One way backlink : Site A ------(Backlink)----- > Your Site Explanation : The site A is giving a backlink to your site. You don't need to pay back with a backlink to Site A Two way backlink / Reciprocal Link : Site A <------(Backlinks)----- > Your Site Explanation : Both the site is giving backlink to each other Three way backlinks: Site A -------(Backlinks)-------> Your Site ^ ^ ^ (Backlinks) ^ Your Secondary Site Explanation : The site A is giving a backlink to your site. You are paying back with a backlink from your secondary site so that your main site stays clean. There is a four way link exchange scheme exists also. Same as 3 way link exchange, site A is also has an optional site to give backlinks to get one P.S: Only one way backlinks are safe. Any other link-schemes may harm your website presence in SERP
You just need to know that you are not doing two-way link building (reciprocal link building).... and almost all the methods of link building is one way..except reciprocal link building.