have a handle on (something)
To have a firm, clear understanding or determination of something. I hope everyone has a handle on the material, as you'll be tested on it next week. I thought I had a pretty good handle on English idioms, but I don't understand this one at all!
get/have a handle on somebody/something
(informal) become/be familiar with and so understand somebody/something: I can’t really get a handle on the situation here. What’s happening?