This question has been already answered on ScienceLine.

To go a bit farther than that answer, and getting into why atoms don't move through each other, the particles "push" on each other due to quantum effects which prevent electrons from occupying the same states (the Pauli Exclusion Principle ). While there is electromagnetic repulsion between the particles of the fluid and of the container when they are at a certain distance from each other (because of the repulsion of like charges), this is rather a secondary effect; if one were to push the particles together sufficiently, this electromagnetic repulsion would be overcome and the quantum effects would take over.

Liquids and gasses are fluids, which means that their shape is not rigid and the particles are constantly moving around – a measure of this motion is the temperature of the fluid. As these particles are moving, they collide with the inside walls of the container and bounce off. This collision causes a force to be exerted on the wall of the container as the particle changes direction. The force exerted by the collective group of particle collisions with the container walls is responsible for the pressure that the liquid exerts on the container.

The three states of matter are solid, liquid, and gas. Solids contain atoms that are fixed and do not move. Instead, they vibrate in place. Liquids and gases, however, are made of atoms that are in constant random motion, flying around and bouncing off of whatever container they are in. Temperature makes them move faster, so gas particles move faster than liquid particles. Let’s say you have a water balloon, the pressure of water molecules bouncing off of the inside of the balloon is what keeps it from collapsing. When you squeeze and compress the balloon, you decrease the amount of space the atoms have to move around, so they bounce off of the walls harder and can result in higher pressure, causing the balloon to pop.

The same way that the particles of a solid do, except that the liquid, being flexible, exerts pressure on the sides of the container as well as on the bottom (although non-rigid solid material, such as a jar full of marbles, will also exert pressure on the sides). Normal force pushes the liquid molecules upward and outward, at which point they push against the walls of the container.

Liquids are made of really small particles called molecules. Let's take water for example. In a glass of water, there are so many small water molecules that you cannot see them with your naked eye. You can only perceive this collection of water molecules as one continuous stuff. This is similar to looking at a nice digital photo---but when you zoom in the photo and look very closely, you should see that the digital photo is made of many small squares called pixels. Unlike the pixels of a photo, however, the molecules of water are always moving around randomly. Why? It's because they have so much energy and nothing is locking them up in place, so they are free to move around. Because their movements are random, these molecules collide with the walls of the container. When the molecules collide with the wall, they exert a force on the wall. Pressure is the sum of all these forces per area of the container wall.