Usually the term "effortless effort" is associated with Taoist philosophy and its concept of "Wu Wei". It has to do with how we act, or experience action, in daily life. I like the Wikipedia description of Wu Wei as "natural action" giving the example of a tree growing. It is doing growing, and yet it is not doing it.
I like to use "effortless effort" when talking about how to meditate. It's indicates that the art of meditation is not one of following instructions. It's the art of allowing the mind to experience a natural state.
I often tell people not to take what I say in my meditations too literally. Sometimes I am asked what I mean by something like "not minding thoughts". It's impossible to answer those questions. The words I use aren't meant to be instructions to follow precisely. The words are more like confirmations of the correct experience. Quite naturally the mind will start relaxing into a state of "not minding thoughts", and if there is some resistance to that happening, words can give you permission to let go. My words are more like "reminders" to gently prompt the mind to let go of effort. But that letting go is an effortless effort!
How can effort be effortless? It's a paradox. The paradox happens because in guiding someone in meditation, we pretty much have to use words. You can't demonstrate meditation like you can dance, because it's an internal process. Although music alone can sometimes induce a meditative state, more often than not some verbal guidance is necessary. And yet, using words and phrases to guide that process is full of pitfalls. The meditative state is actually something that the mind falls into, not something you can make happen through following instructions. The instructions can only set up a situation where the mind can slip into that state.
Meditation is a state of effortlessness and sometimes a phrase here and there can help us to let go of effort. For example, I might say "let thoughts go". The idea isn't to actively let them go, like when you open your hand to drop a ball, but rather hearing the phrase "let thoughts go" may help the mind let go. That's because the mind is naturally drawn into a meditative state when given the opportunity, and there may be some resistance to that happening. The words can help dissolve the resistance. Letting go is not an active doing. No words or concepts can tell you exactly how to do it.