Weinre is a tool that allows remote debugging of mobile web apps by inserting custom JavaScript into the target app and connecting a client. It works by having the target generate debug events that are sent to and evaluated by the server, which then sends responses back to update the client's WebKit Inspector. While Weinre is useful for mobile environments, running a local server can be inconvenient, so services like debug.phonegap.com and built-in browser tools are better alternatives.