Anti-Virus programs (also called anti-malware, virus scanners or end-point protection) are programs that help prevent your computer system, mobile phone, or tablet from becoming infected with malware.
Most anti-virus programs work by comparing (a.k.a. scanning) files against a database of known types of malware. Because malware is continually being developed by cyber-criminals, the database needs to be updated in order to identify new types of malware. The database is updated by downloading periodic updates called "definition files". If you are using a subscription based anti-virus program, and you let your renewal lapse you will stop receiving these "def file" updates. This means your system and files will be vulnerable to new types of malware.
A good anti-virus program is absolutely essential for your computer (and yes that includes MAC systems). However for it to be effective, it must be able to get definition files on a daily basis (hourly is better) and the real-time scanning feature must be active and enabled.
The downside is that because anti-virus programs primarily rely on comparing files to known viruses.. when a brand new virus is "released into the wild" it can take hours and sometimes days for the anti-virus program to be able to recognize it as harmful malware. For this reason, do not rely on your anti-virus program alone to detect "bad" files. You must use your judgment and be cautious about opening files and email attachments. Even when they come from someone you know, be careful. Email accounts and web pages are frequently hijacked to take advantage of your familiarity with the source to get you to open and run infected attachments and downloads.
A good resource you can use if you aren't sure if a file or weblink is "good", is to check it against VirusTotal. VirusTotal is a project managed by a consortium of groups that takes a file you upload to their website and scans it using multiple commercial anti-virus programs. Still not a foolproof guarantee that the file is safe, but this is a great resource for checking files and weblinks. Their website is at virustotal.com
Starting with Windows versions 8 and 10, Microsoft provides a free and full featured anti-virus solution called Windows Defender. Users with Windows 7 may have a version of Windows Defender that is not full featured, and will need to add Microsoft's "Security Essentials" program (available for free from Microsoft's website) to have real-time protection. Windows versions older than 7 will require a third party anti-virus solution. BitDefender and MalwareBytes provide endpoint protection products that are some of the industry leaders in real-time detection of malware.
While malware for MACs is not as common as with Windows, your Apple based system is not immune to specifically crafted ransomware and viruses. Avira and Malwarebytes are vendors that offer free anti-virus programs for your Mac OS based system.