Half Blood is a term for a witch or wizard that has 1 magic parent and 1 Muggle parent. Most of the wizarding world is either made up of half bloods or muggle-borns. There is no derogatory term for half bloods like there is one for muggle-borns (mudbloods). So the purebloods that think they are better than everyone else, can't call them a bad name.
Seamus: im half-and-half. dad's a muggle. mum's a witch.
Dean: yeah im half blood. my mum's a muggle and my dad's a wizard.