Margin is a short term loan from your broker that requires interest payments. As noted in the boglehead article, usually this requires borrowing against your portfolio (or if you're using Robinhood, you flip the switch that allows margin and borrow against a software bug). The brokerages have high speed computers programmed with people who like money to make sure anything risky you do on margin doesn't come back on them, and they can/will sell shares out of your portfolio to balance their risk.

If you want more exposure than you have cash, there are funds like $NTSX that use witchcraft to effectively capture 90% of the 500 largest stocks in the US (saying S&P 500 is trademarked or something) and 60% of the bond market, for a total of 150%. I don't know much more about it except to say that if the fund goes to zero, which everything could, your exposure is limited to the money you put into it.

Note that none of this is investment advice, and any cash you stick in a mattress loses purchasing power to inflation.

Sources:

• https://www.bogleheads.org/wiki/Margin
• https://www.youtube.com/watch?v=d80ahvRSV8E (a man loses a bunch of money on stock options that he didn't have and livestreamed it for some reason)