What does "Build up EQUITY" actually mean?

Let's say for example:
If I bought a house 5 years ago at a cost of $300K. (Paid $60K in cash for down payment, $240K by mortgage loan)

Over the past 5 years, I repaid my mortgage $90K ($15K towards Principal; $75K towards Insterst)

The the market value of the house increases to $315K.

How much Equity have I built up?

Is it $90K? I'm just not so sure about it, though.

your equity build up is the appreciation of the value of your house which is $315-$300k =$15k PLUS the amount of principal you paid down on your loan of $15k according to your numbers.

therefore you have built-up equity in the house in the amount of $15k+$15k=$30k.
your equity is $30k higher.
your total equity is now $90k.

equity is the current value of the house minus the remaining principle owed.

315 - 225 = 90

you have 90K in euqity

You only build equity if the percentage by which the price of the house goes up is more than the percentage of the interest you pay on the loan.

Equity is the value that is locked up in your home. It's the amount of the proceeds that you would get if you sold it at any point in time. So when you started, you had $60K in equity. Over the years, you increased your equity by $15K by paying it down, and by another $15K because the value went up. So you have 'built up' an additional $30K in equity. Your total equity at this point would be $90K.

Given the figures you mention, the mortgage balance would be down to $225k. If the FMV is now $315k your equity would be $90k.

Another way to calculate it would be to add your original equity of $60k to the $15k in FMV increase and the $15k in mortgage balance decrease. The result will be the same.