Two meanings:
1. Borrowing Money: An asset (like a house or car) that you promise to a lender, so in case you fail to repay the loan the lender can take that asset.
Example: you borrow $10,000 and use your car as security for the loan. The lender can take your car if you don't repay the loan.
2. Investing: a tradable asset such as stocks (shares in a company), bonds (loans to a company or government) etc. They can be bought and sold.
Example: Buying stock in Microsoft means you own a small part of the company, while buying a government bond means you're lending money to the government.
Example: this old bond is a type of security:
