It means "independent" but its usage is much narrower than that of independent. As with most synonyms, they are not interchangeable and you have to study example sentences, definitions in monolingual dictionaries, and find them in context if you want to understand them well.
"Stand-alone" is often used in the context of computer networks and other networked systems. A stand-alone PC is not connected to the network, often for reasons of security and confidentiality. Businesses might set up stand-alone systems, properties, or units which are self-contained and not connected to the main systems of the business. These things "stand alone". You can also have a "stand-alone" event which is not part of a series.
See examples here https://fraze.it/n_search.jsp?q=stand-alone&l=0 (Look at the sentences with a hyphen (-) "stand-alone").