保票 literally means 'guarantee certificate', but in everyday Chinese it is almost always used in the fixed verb-object phrase 打保票 (dǎ bǎo piào), meaning 'to guarantee' or 'to vouch for something'. You can think of it as 'to give a guarantee' — it's not a physical document but a figurative promise. The noun alone is rarely used outside this pattern, so focus on mastering 打保票.