Something effortless is easy to do — or at least appears that way. Great basketball players make scoring three-point shots look effortless. If you always know just what to wear, you have an effortless sense of style.
Effort is the work it takes to do something. Moving a couch and staying on a diet both require effort. If something is effortless, it takes no work at all to do. For example, lifting a feather is effortless. Writing may be effortless for some people, but difficult for others. Also, some things just look effortless. A great musician might make playing guitar sound effortless, but years of practice probably went into it. Experts make difficult things seem effortless.