Its way easier to understand if you experience it in person. Try it at home with a cup next time you wash the dishes; put a cup entirely underwater and let all the air out, turn it upside down so that the open top of the cup faces down, with the cup still under water with no air in it start to slowly pull it up out of the water bottom first. The cup will remain full of water even though you pull it above the surface, only when the opening is lifted above the water and air can enter does it empty. Feel how the cup resists being pulled up, it "wants" to equalize but can't as the lack of air in the cup is making a vacuum.

The same happens with a cup full of air. Turn an empty cup upside down above water, then being careful to keep keep the cup straight vertically and keeping the air in push it below the surface. The air will stay inside the cup, you will feel the water "wanting" to be in the cup and push the cup upwards, if you were to let go it would shoot to the surface like a rocket.

After that it will be way easier to understand the explanations people are giving where they go into atmospheric pressure, water tension, gravity, etc.