what is stress
A bar is being pulled from both ends. From the outside you see only the two forces at the ends. Inside the bar, every cross-section is carrying that force. To see it, you imagine cutting the bar once. On the cut face the internal force that was joining the two sides shows itself. Stress is the amount of that internal force carried by one patch of area. It is not the force itself. It is the density of the force. This one point of view is where mechanics of materials starts. It is why the same force breaks the thin place first. The thin section reaches its limit first under the same force.
Pull the bar first. Move P and the bar lengthens, and a larger pull lengthens it more. That is tension, the state of pulling the two ends in opposite directions. The wall holds one end, so the wall pulls back with the same size, and that return is the reaction. For now, only the feel matters. The force is passing through the bar, and the bar is holding against it. The next scene opens the inside of that holding. The holding already lives inside the bar.
Now cut the bar in your mind. You are not really cutting it. You are asking what would show if you cut it here. Look at only the right piece. P pulls at the end, yet the piece does not fly off. The left piece was holding it with the same force on the cut face. That holding force is the internal force. Slide the cut and, wherever you cut, the internal force is always equal to P. The external force runs straight through the bar. This imagined cut is how you see the inside. Move the cut and the force inside stays the same. The place of the cut does not change it.
The same internal force feels different in a thick bar and a thin one. The whole cross-section shares the internal force P, so a wider area leaves a smaller share for one patch. That share is the stress, σ = PA. Move the area A. P stays the same, and when A widens the color goes to a low stress, and when A narrows it goes to a high stress. Pull the same way and the thin part becomes dangerous first, because only there does the stress climb. What decides a break is not the force. It is the stress. Area is what divides the share of one patch, and that share is what the color is showing.
How the force meets the cut face matters too, and there are two ways. If the force is perpendicular to the face, it pulls the face straight apart or presses it, and that is the normal stress σ. Pulling is tension and pushing is compression. If the force runs parallel to the face, it slides the face, and that is the shear stress τ. Switch the two with the buttons. Normal opens the block, and shear shoves the block sideways. Both are force divided by area, but the direction differs, so the way a material holds and the way it breaks differ too. When a bolt or a pin snaps, this shear is usually the cause. Direction decides the shape of the break.
Now a sense of the unit. Stress is force divided by area, so the unit is N/m², and that is called the pascal, Pa. Pa is far too small, so engineering uses the megapascal, MPa, a million times larger. The relation worth keeping is 1 MPa = 1 N/mm². That is 1 newton on 1 square millimeter. Move P while pulling a steel bar and you see where the stress sits on the scale. Atmospheric pressure is about 0.1 MPa, concrete is tens of MPa, and steel yields around 250 MPa. Attach this sense of place to the numbers and you can tell at once whether a result makes sense.