vSphere DRS can be enabled when creating a new vSphere cluster, or by editing the settings of an existing cluster:

When DRS is enabled, the DRS Automation Level and Migration Threshold are set to determine how DRS will place and migrate virtual machines between hosts in the cluster to balance the resources across all hosts in the cluster.

If the Automation Level is set to Manual, vCenter will make suggestions for initial virtual machine placement and virtual machine migrations. When a virtual machine is powered on, DRS makes a suggestion for the initial placement of the virtual machine based on the balance of cluster resources, but this must be acknowledged by (or can be changed by) the administrator. Migrations will not be performed unless they are acknowledged by an administrator.

Setting the Automation Level to Partially Automated will make vCenter automatically select a cluster host to place the virtual machine at power-on, but it will only make recommendations for virtual machine migrations. Migrations are not performed unless they are acknowledged by an administrator.

When the Automation Level is set to Fully Automated, it allows vCenter to automatically determine the initial placement of virtual machines. This setting also causes vCenter to automatically migrate virtual machines between the hosts in the cluster, in order to balance resource usage across all cluster hosts. When the Automation Level is set to Fully Automated, virtual machines will also automatically be migrated to other hosts in the cluster when a host is placed in the maintenance mode.

The Migration Threshold determines how the cluster will be balanced when the Automation Level is set to Fully Automated, or how DRS recommendations will be generated when the Automation Level is set to Manual or Partially Automated.

A conservative migration threshold setting will only cause virtual machines to migrate if the migration will result in a significant improvement in the balance of resources. Setting the migration threshold to be more aggressive will cause the virtual machines to migrate if any benefit can be realized from the migration. Setting the migration threshold to be too aggressive can result in unnecessary virtual machine migrations, or virtual machines constantly migrating in an attempt to aggressively balance the resources.