The "spatial join" is an operation that allow to combine the attributes of two geometric layers when  they meet a given topological condition (for example when the feature of one in contained by a feature of the other).
This is a basic function of many GIS systems. In QGIS there is a command in processing toolbox or in vector menu called "Join by location" that allow to create a new layer from the intersection of two existing layers.
The new plugin called "SpatialJoin" is aimed to perform spatial joins without creating a new table, using the full set of available tolological condition. This is done exploiting the new expression fields feature, available from release 2.6.
The plugin, using the "refFunction" expressions extension, needed as dependency, creates a expression spatial key field ("spjoin_rif") based on reference functions and joins the desidered fields with a value by id function in expression fields as well.
The result is a complete dynamic table that resolvs spatial joins on the fly, evaluating the current layers features. All the plugin newly added fields  are signed with a "spjoin_" suffix
Dynamic joins are very computational insensive, so it's recomended to test them with small datasets to avoid long waiting for results. 
As alternative it's possible, unchecking the dynamic join option, to create a new static field to improve performance. 
Furthermore, there is a last experimental option that allows to build up relation based on the two table join.