What does -o- -moz- -webkit- mean before display: flex;?

Question

css

I copied and pasted the code, but didn't understand anything. What are those things before display: flex;?

spoiler

.d-flex {
  display: -webkit-flex;
  display: -moz-flex;
  display: -ms-flex;
  display: -o-flex;
  display: flex;
}

Answer 1

https://developer.mozilla.org/en-US/docs/Glossary/...
Browser vendors sometimes add prefixes to experimental or nonstandard CSS properties and JavaScript APIs, so developers can experiment with new ideas while—in theory—preventing their experiments from being relied upon and then breaking web developers' code during the standardization process. Developers should wait to include the unprefixed property until browser behavior is standardized.
Browser vendors are working to stop using vendor prefixes for experimental features. Web developers have been using them on production