Without any context it's impossible to say. It could mean anything. I remember I got into chess when I lived in Brasília and played at the clube de xadrez and a buddy and I were watching a friend play (he was a much stronger player than either of us). The guy we were watching was taking far too long as we thought he had a very obvious winning move. My friend said, "Tenho certeza que a gente errou." Meaning "I'm sure we got it wrong." The "it" in this case was implied (o cálculo or the calculation of the moves). And he was right. When we asked our other buddy about it, he showed us that if he had made the move that we thought won the game (he would have gotten a bishop and night for 2 pawns) it would have left him open to a checkmate on the next move. In this case there was no need to say "Tenho certeza que a gente errou o cálculo." because it was obvious from the context what he was talking about.