Portfolio Margin

In general, margins refer to loans you take out to buy stock (or other securities). Your broker will lend you cash so you can invest it in the market. Like getting a marker at a casino.

Margins for stock trading are relatively simple. The process gets more complicated in the derivatives market (where things like options and futures are traded). Valuations of products become more complicated in that market, and the use of expiration dates make unraveling positions in a short period of time (as is necessary during a margin call) more complex.

That's where portfolio margin comes into play. It's a way of computing the margin requirements for a derivative account.
Instead of looking at individual positions (say, a call for 100 shares of XOM at $75 a share, expiring in three months), it looks at the net situation for the portfolio as a whole. So it takes all positions into account.

You might be underwater with that XOM call, but you may be in-the-money for the put you have for NFLX. The NFLX put offsets the loss you're posting in the XOM call, thus your portfolio is positive overall.