What is Zen? This question is as vast as ten oceans with ten directions, but it starts and ends with exactly where you are. You can arrive by breathing.
If you'd like to practice Zen, turn the lights down. If you can sit in full lotus, do it, if not, half-lotus, if not that, then burmese, if not that on your knees. In all of these seats, put a cushion directly under your sit bones, lifting your pelvis above your knees; this is critical. If you can't sit in these positions, get a chair and slide to the edge of it. Firmly plant your feet. If you're on the cushion, firmly plant your knees. Make sure your spine is relaxed, but straight. This is critical. Press the sky with your head, stretch the backbone. Let your eyelids relax, but keep the eyes open, lest you invite dreams and nightmares. Gaze softly downward, a few feet in front of you. Relax the face, like a baby's face. Relax the shoulders and rest your hands on your knees, or place them in the universal mudra, like holding an fragile egg. Now breath.
Start with a quick inhalation, fill the lungs, and let the breath out slowly, feel it rise from below your belly, extend it as long as you can maybe thirty seconds. Repeat. Do this to settle
Now let the long breaths be long and short breaths be short. If you get lost, or scared, or irritated, go back to quick inhalation, long exhalation. When thoughts arise, ask "What is this?" and answer with "What is this?"
This is just what I do today. I've done many other things. Zen is successfully elusive every time, please encourage it to be so. Since you asked what is Zen, that's good enough, it's maybe the best it will get. Find a teacher, find some dharma friends, if this feels right.