。
We were about to play baseball.
Literal
We [topic-は] baseball [object-を] do-volitional [trying-としていました].
The volitional+としていた pattern expresses 'was about to [verb], was trying to [verb].' しようとしていた = 'was about to do.' The progressive past indicates an action in preparation or attempt when something interrupted. 僕たち is the plural 'we' formed with the inclusive ~たち suffix.