In geometry, "construct" means do it with a straightedge (not a ruler) and compass.
Here's how you do it:
1. Using the straightedge, draw a line clearly longer than segment RS.
2. Place a dot at one end of the line you drew.
3. Stick the "center" of your compass in the R position of the RS segment.
4. Open the compass so the pencil point passes through the S position. Your compass now marks off segment RS.
5. Without opening or closing the compass, pick it up and stick the center into the dot you made on your new line.
6. Still without opening or closing the compass, make a hash mark that crosses your new line.
7. Place a dot where the hash mark crosses the line. The segment on the new line between the two dots is a segment congruent to RS.
Notice that you didn't use a ruler to measure anything. This is called a "construction" in geometry. It's what the ancient Greeks used to do.
