This sweet potato is undercooked and has a gritty texture.
Literal
This sweet-potato [topic-は] half-cooked [copula-で] crunchy-do.
薩摩芋 (さつまいも) 'sweet potato' literally 'Satsuma (province) potato.' 生焼け 'half-cooked / undercooked' is a compound of 生 'raw' + 焼け 'baked.' がりがり is a mimetic for a hard, crunchy, gritty texture — the unpleasant crunch of something that should be soft.