Generally, the word Samadhi is translated as “concentration” in English in the context of Buddhism. There are longer explanations that are used as well, that go beyond just a single word. It is meant to refer to a state of deep concentration in meditation, with “right concentration” being a state of meditative concentration so well developed that it’s the final stage before enlightenment. This is where your mind is so concentrated that you’re able to see things as they really are.
I recently encountered an alternate take on this that I’ve found very useful. I don’t remember at this moment if it was in one of his books or one of his talks, but Ajahn Brahm suggested that you think of it less as “concentration” and more as “stillness”. This slight shift in perspective has made a difference in my meditation. Focusing on stillness has brought me, rather effortlessly, into longer and deeper meditations. I meditated for an hour and 15 minutes today and barely noticed the amount of time that had passed. I might have gone longer had I not been kind of brought out of meditation by a feeling of hunger.
Brahm says that when you are still enough in meditation, the sensations of the body fade away. You will stop doing the million little things we do, like scratching our noses or shifting our posture, to get physically comfortable. You won’t be distracted by sounds or smells. A year ago, I’d have not believed I could be that “concentrated” even after some of the things I’ve experienced in meditation. It hasn’t happened yet, but today, I can actually imagine that it might.