I have seen a few questions over the years by players just upgrading to a new monitor with higher resolution and wondering why they still see jaggies and need anti-aliasing. It is always a similar story:
I just bought a new 21.5″ monitor and 1920×1080 resolution. My old monitor was only 1024×768. I thought people said you don’t see jaggies with this high resolution. Is something wrong with my monitor?
The pixels on a typical monitor are pretty small. You would think we would not be able to see them while sitting two or three feet away from the monitor. Well the individual pixels are hard to see, but jagged edges are not. They are most common when a line drawn on the screen is near vertical or horizontal but not quite. Our eyes are able to pick up the stair-steps in the line pretty easily.
Anti-aliasing (multisampling in computer graphics) is a way to smooth this out by rendering jagged parts of the image at higher resolutions (2x, 4x, etc.). Then scaling those higher resolution parts back down to the original resolution. This blurs the edges of all the lines just a tad and covers up a lot of jaggedness in the final image. At first glance it would seem we just need higher resolution monitors, but it is a little more complicated than that.