The meaning of these things are obvious to the speakers because they have heard them since childhood, and know the stories and jokes they refer to or come from. If you think about your own language, you will find many examples of things you say every day that would make no sense to someone who only knew the meanings of each word in the sayings and typical phrases you use. Speaking of which, That's how it goes is one of a large number of expressions that mean that life is difficult - you won't always get what you want, or won't get it easily, and that you had better get used to it.
Tables are part of daily life, so they show up in a lot of sayings and expressions.
To turn the tables is to change a situation that was not in one's favor into one that is in one's favor. Business done under the table is sneaky or illegal, and most of all hidden. Imagine someone passing a wad of money to someone under the table so no-one can see what's going on. This may help you remember that legitimate, legal activitity is above-board . If you go to a meeting, what you bring to the table is your contribution - what you've prepared ahead of time, or what you can add to the meeting.