Resolution is the measure in which how many pixels are displayed on the screen.
For emulation of 2D systems, the resolution can only be upscaled, making the pixels more apparent. For emulation of 5th generation consoles and newer, the internal resolution can be increased to make the game look sharper.
|Master System||256×192, 256×224|
|Game Boy/Color, Game Gear||160×144|
|Nintendo 64||640×240, 640×480**|
|Playstation||256×224p, 256x240p, 320x224p, 320×240p, 512x224p, 512×240p, 640x224p, 640x240p
320x448i, 320x480i, 370x448i, 370x480i, 512x448i, 512x480i, 640x448i, 640×480i
|Game Boy Advance||240×160|
|PlayStation 2||512×224 512×448
608×456 640×480*** etc
|3DS||800x240 top screen****
320x240 bottom screen
*This is a rough figure given for simplicity's sake. In reality, the Atari 2600 doesn't really output pixels, and it has no limits on the number of lines it can display. However, it did have a hard limit on the number of horizontal color clocks for drawing the picture (160), and most games only output 192 lines, hence the commonly given resolution of 160x192.
**While N64 games ran at various resolutions internally, in practice the hardware's VI component always doubled the scale horizontally, and output in either 640x240p or 640x480i, though there is letterboxing at times.
***Similar to N64, games ran at various resolutions internally, though output is usually in 480p.
****This is the "true" resolution of the top screen and what games will be rendered at in full 3d mode, however, due to said 3d effect the horizontal resolution is effectively halved. Each eye will only see 400x240 and games run in 2d mode will (normally) be rendered at 400x240 
- Main article: Scaling
Upscaling the resolution will only look good if you scale it by integers (2x, 3x, 4x, etc.). If you are scaling with non-integers, you can make the image look better using the Pixellate shader.