It's very simple. A traditional balance sheet has two columns which have to balance. The left side, usually marked ASSETS, has to equal the right column, traditionally marked LIABILITIES.
Both columns have to be equal, under all conditions, no matter what.
This confuses people. They say, "oh gosh, I have 20,000 in debt (liabilities) and 300,000 in the bank (assets), so how can assets equal liabilities?"
So to solve this problem, accountants put another number on the right hand side, called EQUITY. It's a made up number... you derive it by taking ASSETS-LIABILITIES.
Thus, ASSETS = LIABILITIES + EQUITY.
And if anyone ever asks what EQUITY means, there are two possible explanations:
What is Equity? It doesn't matter. It's just a made up number that you put in so that the bottom number in the ASSETS column is exactly equal to the bottom number in the LIABILITIES column, because we like to balance the balance sheet.
No, really. What is Equity? It's the amount that the owner of the company owns, by virtue of owning a company with more assets than liabilities.
Now, let's look at your figures.
You said that $300,000 is your total assets. We got that by adding up the value of everything your company owns. Easy.
You said that $20,000 are your liabilities. We got that by adding up the value of everything that your company owes. Also easy.
But! If we only had $20,000 of liabilities, then the balance sheet isn't in balance! Danger Will Robinson!
So we made up a number called Owner's Equity to include in the right hand column, and we made it $280,000, because that makes the whole thing balance, and there are two interpretations of this:
It's an imaginary number there to make the two columns add up, or
It's the "book value" of the company -- the amount that YOUR OWNERSHIP in the company is worth based on the books.
One last thing
QuickBooks does something tricky: they split up the Owner's Equity into two pieces.
Net Income ($30,000) is this year's profit, so far. It's the amount that Owner's Equity went up this year.
Retained Earnings ($250,000) is all previous year's profit that has never been paid out to the owners as a dividend. It's the amount that Owner's Equity went up before this year minus any money which was taken out as a dividend, which is no longer in the company and no longer on the balance sheet. It's also the owners equity minus this year's profit.
QuickBooks is doing this so you can quickly see this year's profit from the balance sheet, but it's not the most common thing in the world outside of the QuickBooks universe.