Imagine your house is full of holes. Look to those hole everyday bothers you and you need to do something to close them. It can vary, but calling a builder, using plaster, spackle, for example, probably the problem will be solved.
When we don't update with frequency operational systems and softwares that we use, can be a laptop, tablet, smartphone, TV or any other device, we are putting them at risk, leaving them full of "holes", which technically known as vulnerabilities.
These vulnerabilities can be exploited by malicious individuals enabling remote access if exploited, datas theft, resource compromise and many other possibilites.
The vulnerabilites are identified and cataloged using a CVE number (acronomy to Common Vulnerabilities and Exposures) and information about them can be easily found on websites like https://nvd.nist.gov/vuln/search/ ,  https://cve.mitre.org e opencve.io.
Obviously updates are not a silver bullet but surely reduce your exposure to the risk that these vulnerabilites can bring. Therefore, next time you receive a notification to update your device, please, do not cancel them. :)