The succinct description from the property itself is "Returns the entity type of the object as an integer". Example, lines are 19, arcs are 4, circles 8 etc. Obviously they are members of an enumeration tho it appears not formally. That is, while I found the enumeration AcEntityName, with values confirming the preceding "lines are 19" etc. the return type for the EntityType property is not spec'd as AcEntityName but merely Long:
Property EntityType As Long
read-only
Member of AutoCAD.AcadLine
Returns the entity type of the object as an integer.