Definition
Friction is the force that pushes back when two surfaces touch and try to move, or slide, against each other. Thanks to this invisible force, we can walk and run without slipping, and a speeding car can stop safely right where it should.
Why does a moving object stop on its own?
Even a smartphone screen or a window that looks perfectly smooth turns into a jagged mountain range under a powerful microscope. When two objects touch, these tiny bumps (little peaks and valleys) lock together like the teeth of gears.
To push one object forward, you have to break those locked bumps or climb over them. That effort creates a braking force at the contact surface. This is friction.
Friction doesn't point opposite to where the object is going. It acts against the direction the two touching surfaces are trying to slide. That's why a sled sliding on ice, or a box you shoved across the floor, gets pushed backward as it scrapes forward and slowly comes to a stop. When you walk, though, your foot pushes the ground backward, so friction actually pushes you forward.
Starting and sliding take different amounts of force
Have you ever tried pushing a heavy bookcase or refrigerator across the room? Getting it to budge takes everything you've got. But once it starts moving, it slides along with much less effort.
In physics, the force that holds back an object at rest is called "static friction." The force on an object that's already sliding is called "kinetic friction." When an object sits still, the bumps on the two surfaces settle deep into each other's gaps and lock tightly. So the resistance at the moment you start pushing is the biggest.
Once the object moves at a steady speed, the bumps zip over each other's tops without time to lock in deeply. So the friction you feel while it's moving is smaller than the hurdle you had to clear to get it going.
A bit more precisely
Common sense says more contact area should mean much more friction. Pushing a brick lying flat feels like it should be harder than pushing it on its end. But to be more precise, the friction between solids doesn't depend on how wide the contact area is.
When the contact area gets bigger, the total weight spreads over a wider surface, so the pressure on the ground goes down. When the area gets smaller, the weight concentrates on a small spot and presses harder. In the end, the "true microscopic contact area," the total of the points that actually touch and grab each other, comes out about the same in both cases.
What's more, if you press two very smooth metal surfaces together in a vacuum so they touch perfectly, they stick hard to each other even with no bumps. Friction isn't just rough surfaces catching on each other. It also comes from the electrical attraction between atoms and molecules pulling on each other. Soft materials like tire rubber break this rule, though, and can be affected by contact area.
Common misconceptions
The more surface area an object has touching the ground, the more friction there always is.
For hard solids, friction has almost nothing to do with the apparent contact area. It's roughly set by how hard the object presses straight down and by the material and condition of the two surfaces.
Friction just wastes energy and gets in the way. It's a useless force.
Without friction, we couldn't step forward on the ground, we couldn't write with a pencil, and the thread in our clothes would come untied.
Where you meet it
Sprinkling sand on a snowy road makes the surface rough so cars slip less (calcium chloride helps by melting the ice).
The deep rubber grooves on the sole of a shoe keep just enough friction with the ground to prevent slipping.
A car's brake pads clamp hard onto the spinning disc, and friction brings the car to a stop.
In one sentence
Friction is the force that resists motion, caused by tiny bumps on touching surfaces and attraction between atoms, and it makes everyday actions like walking and stopping possible.