Two meanings:
1. When you borrow money, the "security" is the asset (such as a house or car) that you will lose if you can't pay back the money.
Example: you borrow $10,000 and your car is the security for the loan. If you don't pay back the money, then the lender can take your car.
2. A general term for stocks, bonds and other types of legal documents that can be bought and sold. They usually represent loans or shares in a company, government, or other organization.
Example: this old bond is a type of security:
