Digital singnage consists of hardware (electronic screens - LCD, plasma, office projectors etc .. - that you can find outdoors, or inside shopping malls, airports and so on, that communicate ads and brand awareness, as well as general information, to consumers) and software (anything from free software or powerpoint to expensive software used for large digital signage projects).
Digital signage has not always been the most important tool for the digital media planner (it can be expensive and difficult to judge its success) however with technology improving, and prices dropping, digital signage is certainly becoming more popular, and increasingly, an interesting option in digital media planning. It's great plus point is that it offers the digital media planner a way of communicating campaigns and brand values to consumers in an interactive way (and the technology is improving all the time here).
For people looking for digital media planner jobs, it is certainly worth spending a little time (relative to other areas of digital media planning, of course) looking at digital signage in general. Digital signage is also a useful subject to look at for people looking for jobs in outdoor advertising.