Here's a breakdown:
* 理解 (rikai) means "understanding"
* して (shite) is the te-form of the verb "suru" (to do), connecting "rikai" to the next part of the sentence
* いない (inai) is the negative form of "iru" (to be), meaning "not".
Putting it all together, "rikai shite inai" translates to "not understanding" or "I don't understand".