By default the value of this property is set to -1 that means that It will be able to run Total Tasks=Number of processors+2.
As we can see in below Fig 1. The SSIS Package is executing 6 Data Flow Tasks as MaxConcurrentExecutables value is set to default(-1). My machine has 4 processors.
If you are sharing your Server to run other application/s , then you might not want to use all the resources for SSIS. In that case, you can set the MaxConcurrentExecutables value to small number.Let's set the MaxConcurrentExecutables=2 in package and run the package and see if it is only executing only two tasks in parallel.
As we can see that only two Tasks are executing in parallel. If you are changing the MaxConcurrentExecutables value from default to some higher number, Test your SSIS Package to make sure you are getting better performance than default setting as well no other application/process is being effected by new SSIS Package setting.
Let's set MaxConcurrentExecutables =10 and see if all the Tasks are executing parallel.