A new baseline has been established for future SharePoint 2013 fixes. This means that you will have to install the Public Updates (PU) that were released on March 12, 2013 for SharePoint Server 2103, SharePoint Foundation 2013, and Project Server 2013 first before installing the upcoming April Cumulative Update (CU).  These can be installed now at the links above, or via Microsoft Update (MU). 

A baseline is a tool to manage the size of the CUs. This means that any fix produced after a given service pack, will not apply to any build prior to said service pack. So, the first baseline is usually the RTM build, the second is the SP1 build, and so on. However, a new baseline is included in the release of the March 2013 PU for SharePoint Foundation 2013, SharePoint Server 2013 and Project Server 2013. This means that any subsequent releases, i.e. April 2013 CU, will not apply to any server that has not had the March PU previously applied.