Ok, lets get this clear. In which table are these columns?
Do you understand what you are putting in that query?
Lets break it down:
The above code says, we are going to take ALL the columns from a table with a shorthand reference of t and and rename the column t.username AS threadusername and a field called desc from a table with a shorthand reference of tfd and two columns from a table with a shorthand reference of u
The next line says that table prefix threads is the table referred to with the letter t. At this point you have identified any field with the t. but the query does not know what you mean by u or tfd. So, we tell it - we say as below:
Now it knows that the tfd shorthand refers to the threadfields table and the u shorthand refers to the users table. But thats not enough - you need to have a field in BOTH tables that are the same - so we say that the field in u.uid is the same value as the field in t.uid. Same with the tfd. We always compare it to the t. as this is the table you have selected from.
Does this help?