Flowers had been placed at the grave as offerings.

Literal

Grave [to-には] offering-flowers [subj-が] offered-[passive-ていた].

供花 (くげ/きょうか) 'offering flowers' and 供える (そなえる) 'to offer/present (to the dead or gods)' share the kanji 供. に...供えられていた is the passive-stative form — 'had been offered.' お墓 uses the polite prefix お. A funeral/grave-visiting scene.