When your design calls for two colors to overlap each other, you will usually knockout the color on top from the color underneath. Trapping refers to a small modification to either the knockout, or to the printing of the top color's plate, so that the two colors overlap slightly when they are printed.
Trapping is necessary because printing presses never produce perfect registration. If two overlapping colors are printed with knockouts on but no trapping, a gap will appear between the two colors at some point in the press run. It will show up as the color of the paper and may be very noticeable. To avoid this, an overlap is set up so that the gap never appears. The overlap must be kept as small as possible, because a darker color is formed in the area of the overlap.
Chokes and spreads are two types of trap and both are supported by RSG. Both terms refer to an adjustment in size of an object, or an object's background. A choke shrinks the open area in the background to create a trap. A spread enlarges the object in the foreground to create a trap. Both chokes and spreads create overlaps. The difference is that for a spread you change the size of the object, while for a choke you change the size of the background.

To decide if a trap is necessary you must analyze your pages. First look for parts of the page where colors touch or overlap each other. If there is no touching or overlapping no choke is needed. Next see if overlapping or adjacent colors, if they are to be printed as process, share any component colors. For example, if red and blue will be printed as process colors, they do not need to be trapped because they will share magenta. Finally, check to see if your pages include fine line work or small serifed type. Trapping will change the size of the image and may affect the overall look of the page.
The optimal size of a trap is a function of the amount of variation expected during the press run. Common trapping values range from .25 to 5 points. One method of estimating proper values for trapping is based on screen ruling. For many printers, good registration on press means being off by no more than half the distance between rows of halftone dots. Thus you could calculate the following chart based on the screening parameters you are using for your job.

The trapping value is very critical. Tool small a trap is ineffective, while too large a trap is noticeable.
With RSG you manually trap overlapping colors using either Pen Trap (from the Draw menu), Fill Trap (from the Draw menu), or Trapping (for text from the Format menu). To determine what to do you must know which of two overlapping colors is darker. If the color on top is darker, you must choke it. If the color on top is lighter, you must spread it. From darkest to lightest, the order of the most common colors is black, red, magenta, green, cyan and yellow.
In RSG trapping is always applied to the top color be it a rule or frame (in which case you would select the object with the pointer tool and choose Pen Trapping from the Draw menu), and interior fill of a graphic object or a text block (in which case you would select the object with the pointer tool and choose Fill Trapping from the Draw menu), or colored text (in which you would select the text with the I-beam tool and choose Trapping from the Format menu).
If the color of the rule, frame, interior fill, or text on top is lighter than the color underneath you should select Spread in the trapping dialog box. If the color of the rule, frame, interior fill, or text on top is darker than the color underneath you should select Choke in the trapping dialog box.

Even though selecting choke will only effect the printing of the object underneath the selected object or text, and not the selected object or text itself, you still apply it to the object or text on top.
A single object may have multiple settings to control trapping. For example, a text block may have an interior fill of one color, the text itself in another color or combination of colors, and a frame in a third color. The text and frame draw on top of the interior fill. Depending on the colors involved, trapping would be set for the text block's frame and text.