In this context, a 'tag' is a piece of metadata - that is data about data.
In our context, where the suggestion is to use tags, you are asked to specifiy data about your own post. The idea is to identify your post with 'tag words' that will help software classify and organize your post with other posts that share the same tags. You may choose a tag that corresponds to no word in your post.
If you write a post about an Ikeda tonearm, tags might be 'Ikeda', 'tonearm', 'Japanese tonearm', 'Mike L', etc.
Without any guidance or training end-user created tags are considered somewhat freeform. They can be a product of our own view of things (preferences), how we think about stuff (ideas, concepts) that a computer might not derive on its own just looking at the words in your post. However most of us are not librarians familiar with thinking about how our posts might be classified and organized for the future. We are not professional taxonomists.
Of course the software can (and does) look at your post's words too - every one of them. But you don't want to index every word in a post. Often this leads to use of what's called a 'controlled vocabulary' which is usually domain specifc. What words are of interest in the domain of audiophiles and high-end audio? The computer uses the controlled vocabulary to rule in or out the relevance of a word in a post. It might see the words such as 'Ikeda' and 'tonearm' as relevant but ignore words like 'joy' or 'axiomatic.' Tags can be created and added; for example 'CES2019' (#CES2019) might be a new tag - it probably already is; Events often create their own tags.
In the wretched world of corporatized social media (Facebook, Twitter, etc. - I'm reluctant to say their names lest they appear) tags are marked with a '#' (hash symbol) and are called 'hashtags'.
The above is super simplistic and not definitive. There are whole realms of people and jobs paying attention to information organization and much of those are sophisticated and complex and sometimes scary. Your tags (and other pieces of information) are used to create a 'picture' of you, what you do, what you like, what you talk about, who you do or don't respond to, where you are, finances, etc. Posting in a public forum means you do not own information about yourself that can be derived from your writing (and tags) and someone else does.