Playtesting is a practice of giving your in-development game (applies to both board and video games) for individuals or groups of people to play so you can learn what works, what needs improvement, how to clarify rules and so on.
Write those notes down. Correct where you need to. And then probably the next night, you’re gonna have to do this whole thing again. And here’s the tragic, heartbreaking part: with a whole new group of people. Because once they’ve seen the instructions, once they’ve seen earlier versions of the game, they are no longer eligible to test a cold read of your instructions. Elan Lee / Board Game Instructions (and why they are the most important thing) by Elan Lee
Board Game Design Lab has a wonderful collection of resources about playtesting at Playtesting - Board Game Design Lab.
Often heard advice about playtesting is to stay in the sidelines and let players figure it out using the rulebook and the components. In that model, designers are advised to take notes and potentially provide answers to questions.
In Pragits, Praktis, Practice. Iteration in game design, James Earnest answers an audience question with a different approach:
Q: Thinking about the very first few playtests, how do you decide whether or not to join as a player?
James: I always play because I’m also learning about the player experience. I’m not really here to observe others. If you want to do a blind test and see if they understand your rules, you should but much later down the road.