Hi Uruj,
If a resource have 8 hours availability in a day and he is allocated for 4 hours only for a Task then you can say that the resource is under-allocated as there are still 4 hours left for the day if you go by the availability of the day.
And if you want to allocate the resource for the same Task and assign 10 hour against him for the day then you will get a pop-up to confirm whether you want to Over-allocate the resource as it is exceeding his normal availability scale.
Hope that helps.
Thanks,
Abhisek Dhar