What does the containment percentage mean when a fire department is reporting on a wildfire?
One hundred percent contained means that there are suitable fire guards around 100 percent of the circumference of the fire.  The fire guards can be either created such as a hand or machine dug firebreak, a retardant break (often placed by air tanker), or a natural feature such as a river or bare rock cliff.  Containing a fire is just a step in controlling the fire, after containment, there are other phases that may or may not be completed such as back burning off the break lines, extinguishment, and finally monitoring.  Even once a fire is 100% contained it is not that uncommon for the fire to "jump the guard" if the weather conditions change.  The percentage of containment also gives an indication as to how much access the fire crews have to the fire, a well-contained fire implies that the fire crew has access to be able to fight the fire, this is the major secondary goal for the dug firebreaks.  

5 percent contained implies that some form of command has been established and the fire crew(s) are just starting to contain the fire.  I wouldn't read much more into the statement.