Translation:In a practice
Mae Ceri'n canu mewn côr, a bydd hi mewn ymarfer yn y neuadd heno - Ceri sings in a choir and she will be at/in a practice/rehearsal in the hall this evening.
Eventually I figured it out. The translation I was given on my phone was "in an exercise," which sounded quite strange to me. I a practice or rehearsal makes perfects sense.