The most simple type of arm in FTC, a single arm refers to an arm on one axis of rotation. While it is possible to successfully build this kind of arm, generally a single axis arm will only afford around 15-16” of extension, which is inadequate for nearly every game.
The reason for this is that the longest a channel can be is 18” (technically you could have a longer channel by placing it diagonally, but this complicates matters). Thus, with a maximum of 18” of extension, a couple inches must be subtracted, since the point of rotation is inside the 18” sizing cube; therefore the extension is around 15-16”.
Therefore, a single arm with further linear extension is optimal. For example, some teams built a single arm with an added linear extension mechanism to reach the desired extension length, which is generally >24”.
The advantages of a single arm are that it is relatively easy to build, and can be a quick way to gain some form of extension outside of the robot cube.
However, there are many disadvantages such as having a high gear ratio, requiring much more support than a linear slide, and being hard to control without the proper software.