Vulnerabilities are bugs in software programs that hackers exploit to infect computers.
Security vulnerabilities leave users susceptible to attacks and can be present in any software product. Responsible software vendors, when aware of the problem, create and issue patches, which address the problem.
There are companies that pay researchers or ethical hackers for new vulnerabilities. There are also hackers that sell new vulnerabilities on the black market. These Zero-day attackers refer to vulnerabilities being exploited before a patch is available.
To avoid vulnerabilities, your operating system and any installed applications need to be running the latest available patches.