妈

Left 女 is the indexing woman radical - a kneeling figure with folded arms, the classic glyph for a female person. Anchors 妈 as a kinship word for a female relative. Same radical family of female-person words: 姐 elder sister, 妹 younger sister, 姑 aunt, 婆 grandmother, 奶 maternal-elder/milk.

Right 马 supplies the sound mā exactly (only the tone shifts - mǎ → mā). Pure phonetic loan; the 'horse' meaning contributes nothing. Same phonetic also yields 吗 question particle, 蚂 ant, 玛 agate. 妈 is one of the cleanest, most beginner-friendly phono-semantic compounds in the language: woman + ma sound = mama.