Dentist said the way my 2 1/5 year old daughters teeth are she is prone to cavaties. But her teeth checked out perfectly healthy and fine. Why is that?

You should ask the. dentist why he/she said that. It is possible that the spacing of her teeth may lead to more accumulation of food, which could lead to more cavities, or he/she could be referring to a surface/enamel issue.
Grooves. may have to do with the grooves on the chewing surfaces of her baby molars. If deep they are susceptible to damage by bacteria because toothbrush bristles can't reach bottom of grooves. Talk to Dentist about sealants, fluoride treatment, brushing technique, flossing, tooth alignment and occlusion, choice of snack foods, avoiding sugary drinks, etc.