。
She tends to underestimate her own abilities.
Literal
She [topic-は] own-self [of-の] ability [object-を] underestimate [tendency-傾向がある].
~傾向がある (tends to / has a tendency to) expresses a habitual pattern. 過小評価する (かしょうひょうかする) means 'to underestimate' — 過小 (excessively small) + 評価 (evaluation). 自分自身の能力 (one's own abilities) is what she undervalues. The sentence describes a psychological pattern — perhaps modesty taken too far, or genuine lack of self-confidence.