DRS will assess where best to place workloads so as they cluster is optimally balanced. You can configure how 'aggressive' DRS is in its recommendations.
Manual means DRS will make recommendations but will not take any action unless you choose to apply those recommendations manually. E.g. DRS recommends Workload A is moved from Host A to Host B.
Partially automated means when powering on a VM (initial placement) it will be assigned to the optimal host. E.g. DRS will automatically choose to place new Workload A on Host B but it will not then move that Workload following that initial placement i.e. while it is running.
Fully automated means DRS's recommendations will be applied automatically. E.g. if host A is over utilised at that time and host B is under-utilised Workload A will be moved (vMotioned online) to host B. This will happen automatically in the background - you will not need to manually review and apply.