Without getting technical, a TLS certificate ( formerly SSL ) is a way to encrypt the data sent from your browser (Chrome, MSIE, Safari, etc.) to the webserver ( google.com, cnn.com, makethemonkeydance.com, etc ).
It both encrypts your data , and proves you are you ( sort of ).
These used to cost up to over several hundred dollars per year, but thanks to pushes from many groups, we now have Letsencrypt that will give you one for free and that's a VERY GOOD THING.
Personally, I always considered paid certificates a bit of a boondoggle and an unspken rip off. You see, you can "sign" your own certificates, but only certificates issued via approved "certificate authorities" would work in your browser without throwing an ugly security warning. Yes, it was a cash cow for nearly 20 years for companies like Thawte, Verisign, and others. I bought several and was never so much as contacted about my business' validity so, yeah, I'm a HUGE Letsencrypt fan.
Large hosting sites, like Shopify, use Letsencrypt, and Wordpress will provide you a certificate so you don't need to worry about it, unlike in the past when it was either a paid upgraid, or you needed to do it yourself.
We need to encrypt data to prevent eavesdropping on your communications with the target website. Your ISP ( Comcast, ATT, Spectrum, etc. ) can listen in, as can any other Internet party in between you and that server. Encrypting the communication removes any temptation as they can no longer snoop. It also prevent bad actors ( hackers ) from injecting bad data into your communication such as malware.
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