What does overhead rate mean?
The 'overhead' generally refers to the funding that doesn't go directly to research expenses (salaries of researchers, cost of experiments/consumables, etc) but to the supporting expenses (salaries of administrative/support staff; cost of facilities and utilities, equipment depreciation) of services that are neccessary but not directly related to research.

It often is set by university administration to some fixed rate (50% sounds high to me, but it depends on country and research domain) that is taken from all grants and used to fund those 'common' expenses.