You use this argument to specify that Dynamic Shared Objects (DSO) are to be used while compiling the modules. All modules except the http_core module and the mod_so module are compiled using DSO. These modules should be statically linked to Apache, exclusively. This is important because the function of the http_core module is to provide core directives for managing the Apache server. The mod_so module is used to enable the server to use DSO modules.
DSO support was built into Apache with version 1.3. DSO allows modules to be enabled or disabled during runtime without the need to relink the Apache kernel. This is particularly useful because disabling the modules results in reducing the size of the Apache executable file. You can, therefore, easily run multiple instances of Apache at a given time using limited memory.