It is a party to welcome someone to a new house. The words don't literally make sense.

A 'house warming party' is a party held soon after moving into a new home. (In some places they have the party on the day of moving) It is an occasion for the hosts to show their new home to family and friends and for the family and friends to give gifts for the house. Some people also invite their new neighbours as a means of getting to know them. The term 'house warming' orginated from times when there was no central heating. Each guest would bring firewood and fires would be lit in all the fireplaces in the house. Apart from the making the house 'warm' this tradition was also believed to repel evil spirits which may have been living in the house when it was empty.

Good gifts include: pota and pans, towels, soaps and shampoos, rugs, cups, things like that. . .

You buy a new apartment or house and you want to celebrate the event, so you invite people over to 'warm' the new place.