That difference matters for risk. A skill can waste your time; a plugin can run code, hold a token, or open a network path. Before installing one, read what it declares it needs, and prefer a version you can pin. If a plugin asks for a credential, scope the credential to a scratch area first.
Plugins are also the part of the stack that breaks most often, because they are tied to the host application's internals. A plugin that worked last month can fail after an upgrade of the host. That is why the directory records the repository and the last update date rather than a promise of support.
Install usually means one command, or one prompt that the agent executes. Either way, verify the plugin is loaded before you point it at real work: ask it for one trivial result you can check by hand. See DeepSeek Harness plugins.