a (good/solid/sound/etc.) grasp of/on (something)
A firm, clear understanding or determination of something. I hope everyone has a solid grasp of the material, as you'll be tested on it next week. I thought I had a pretty good grasp on English idioms, but I don't understand this one at all!
*(solid) grasp of something
and *(sound) grasp of something; *(good) grasp of somethingFig. a firm understanding of something. (*Typically: get ~; have ~; give someone ~.) Try to get a grasp of the basic rules. You don't have a good grasp of the principles yet. John was unable to get a solid grasp of the methods used in his work, and we had to let him go.