Many operators in blender use parameters that control how the operator gets the end result, some of these can be adjusted by moving the mouse as well as pressing keys and typing in values (look in the header as you adjust the operator) before accepting the end result. We also have the option of adjusting these parameters after accepting the result in the operator panel which can also be seen as a floating palette by pressing F6.
For the edge slide operator we can adjust the factor value (you can type this value instead of moving the mouse) as well as having a few options we can turn on/off. The factor is a value between 1.0 and -1.0 that defines the percentage of movement the edge is moved towards the next edge. A factor of 1.0 slides the edges against one edge loop while -1.0 goes to the opposite edge loop, values in between give you a fractional part of that distance.
Using the factor value it is possible to slide two edge loops the same amount towards the next edge. For example, if the edges between the loops outside of a pair of loops are both 1.0 units from the current loop then you can move both 0.7 of the distance and they will then both be 0.3 from the next loop. Effectively this gives the same result as scaling the loops away from each other.
To quickly do that with the keyboard - ⎇ Alt select a loop - GG.7⏎ Enter and then ⎇ Alt select the other loop GG-.7⏎ Enter
While it is common for two parallel loops to be moved opposite amounts (0.7 and -0.7), you can see in this example that sometimes you will want the same value for both loops. If you enter the value manually, repeatedly pressing - will toggle the direction of the slide.
The even option tries to keep the edge parallel to the next edge loop with flipped working with to the opposite edge.