。
She gave up her seat to an elderly woman on the bus.
Literal
She [topic-は] bus [of-の] inside [in-で] elderly-woman [for-のために] seat [object-を] vacated-gave.
席をあける ('to vacate a seat, make a seat available') uses あける ('to open, leave open, make a gap') — the same あける as in 'open a door,' but here applied to space rather than a closed thing. ~てあげる ('do for someone's benefit') marks the action as a kindness done for the elder. ~のために ('for the sake of') frames the recipient. Japanese public-transit culture has a strong norm of yielding seats to the elderly, pregnant women, and disabled passengers — there are even designated 優先席 (priority seats), though giving up one's own seat in non-priority sections is also common practice.