Task Value is an imaginable or sometimes objectively measured worth of a task for completion of a project whereto it belongs. In other words value of a task depends on a contribution that this task makes towards completion of its project. In the most of cases the value of a task can be measured in comparison against other tasks of the same project: for example if you construct a house the works which are related to laying the foundation, building communications, and erecting the walls are obviously more valuable than tasks that refer to finishing and decorating (as first of all you need to have the whole house erected).
Tasks in the projects can be ranked by their values, but these values may have different criteria for appraising – if to speak in terms of time you may consider the tasks residing on the Critical Path as the most valuable ones for the project (when they done on-time the project is completed on-time as well). Task value (expressed in monetary, progress-wise or time-related terms) usually serves as a basis for prioritization of project tasks, so the most impactful activities can be supplied with the best resources and hence to be done at the first place.