Let's Encrypt

Transport Layer Security (TLS) is the protocol involved in getting the lock icon to appear in your browser next to the URL. Under the hood it uses a bunch of really cool numbers for encryption. Some numbers are considered private and need securing; some are considered public and are fine for sharing. You can mix your numbers with other people’s numbers in such a way that you can verify a chain of trust. Ultimately, at the top of this chain there has to be an entity or entities that are implied to be trustworthy, so that the links further down the chain of numbers can inherit that trust. This is the role of a root Certificate Authority (CA) at the top (root) of the chain.