姥 (lǎo) is a colloquial term for 'maternal grandmother' — the mother of your mother. It's less formal than 外婆 (wàipó) and is used more in northern Chinese dialects. You'll also see it in the compound 姥姥 (lǎolao), which is the more common standalone form. This character is rarely used alone in modern Mandarin; it almost always appears in the reduplicated form 姥姥.