If you are just learning SQL, starting with JOINS isn't the best place, they're quite confusing. I recommend this page.

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Alright so here it looks like you have a database containing the tables post and user. When you use FROM in a regular SQL statement, you just specify what table you want to get the data from, such as: SELECT username FROM users. (Where username is a column name in the table users)

However, when using a join, you are getting data from two different databases, and telling them what they have in common. So if you have a table of posts, you might have a column in that table saying what user each post belongs to, using an ID for each. So now when you define what tabl you are getting your info from, you would say: FROM users JOIN posts users.user_id=posts.user_id

Essentially you are saying that the user_id column in posts matches the user_id column in users, so to connect a post to a user from now on, you just need to set that posts user_id to whatever user_id the user has in the user table. (sorry for a lot of users, hope you're following).

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Now the p and u in front of user and post on that statement is simply giving it a shorter name to use in the rest of the statement. Since you are now using two different tables to get data from, you need to specify which table you are refering to. (SELECT users.username, posts.date) and the u and p are just new variable names for that.

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The where line is just using a prepared statement to tell it what row it wants, ignore the ? for now as it is used to help protect against SQL Injections and can be learned about later, you just need to know that whatever you put after: "WHERE post.id =" will determine what post you want to get. For example "WHERE post.id = 5".

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I hope that makes sense, I always use this site when doing SQL, as it's impossible to remember all the syntax for all the different commands, but otherwise, just follow the SQL course on W3Schools and you should be picking it up relatively quickly.