The metaphor I usually go for when it comes to for threads is having several widgets that need to be assembled. To begin working on Widget B, you have to pause the work on Widget A. Adding SMP is like adding a second person to help assemble widgets, but you're still using the same toolbox, so if you both need the same screwdriver, there's no performance benefit to having a second person. Multicore is having multiple workbenches each with their own toolbox so they can operate completely in parallel.
A mutex is like having a specialized tool that can only be used by one widget assembly at a time. Like, each workbench might have a full set of screwdrivers, hammers, wrenches, sockets, etc., but you only have 1 welder.
A semaphore is a tool that can be used by a limited number of widgets, like an oven that can fit up to 4 widgets.