Remember the “loss on paper” from the lesson about owning? That's exactly what equity shows.
An example. You have $1,000 in your account. You opened a trade on bitcoin, and it got more expensive: the trade is now $20 in profit. Your balance is still $1,000, but your equity is $1,020. If bitcoin gets cheaper and the trade drops to a $30 loss, your equity becomes $970. The balance won't change until you close the trade. At the moment you close, the result moves into the balance, and the two numbers match again.