household medicine; medicine kept at home
Refers to medicines routinely kept at home for common ailments, such as cold medicine or stomach medicine.
我が家の常備薬は、風邪薬と胃薬です。
Our household medicines are cold medicine and stomach medicine.
救急箱 refers to the box or case holding emergency supplies, while 常備薬 refers specifically to the medicine kept on hand for regular use, so the two overlap only in purpose, not in form.
Compound of 常備 (じょうび, 'always kept on hand') + 薬 (やく, 'medicine').