A minimum spend pertains to a whole dollar benchmark a customer must meet to secure their chosen space. For example, if the minimum spend for Hotel Bar is $3,000 for a Friday night, the customer must then spend at least $3,000 on food, beverages and in some cases, AV, in order to book that space. In the vast majority of cases, minimum spends can fluctuate according to peak/off-peak periods. Eg: if you're looking to book a private space on a Friday night in December, expect to pay more than you would for, say, a Tuesday night in August.
Hire fees can be charge independently, or in addition to either a minimum spend or per person price. Venues/spaces with 'hire fee only' will typically not include catering. You may be able organise your own catering, enquire with the venue first.