EMZETT.Open TLD
In short: A TLD that anyone can register with no special requirements, e.g. .com, .net or .org.
In more detail: Unlike restricted TLDs, for open TLDs the registry doesn’t check eligibility (no proof of residence, industry, etc.) — whoever registers first gets the name (“first come, first served”).
In Depth
Open TLDs are today by far the largest part of the domain namespace: .com alone covers over 150 million registered domains. The only filter is whether the desired name is still available — registries check neither the identity nor the intent of the registrant, which on the one hand considerably simplifies registration, but on the other also makes abuse easier (e.g. domain squatting: mass-registering names intended to be resold expensively later, or typo domains for phishing).
Classic open generic TLDs (gTLDs):
.com— originally intended for commercial organisations, today effectively used universally..net— originally for network infrastructure operators, also freely usable today..org— originally for non-profit organisations, now also open.
Since ICANN’s expansion starting in 2012, hundreds more open TLDs have been added (.io, .dev, .app, .xyz, and many more), all registerable with no special proof requirement.
See also: TLD, Restricted TLD