I agree with Alicia's comment.
I would like to add a little bit more.
わかる and 理解する are used as synonymous and the meaning is almost equal but sometimes the nuance is slightly different depending on the context.
Usually we use わかる in hiragana.
わかる is translated "to know, to understand" in English.
However this word also includes the connotation of はっきりする(to be clear)、判断（はんだん）する (to judge) 、識別する（to recognize, to discern) and 理解する (to understand, to comprehend) etc...
理解する.......to understand or to seize what is meant correctly, precisely
This is just an example. Sometimes people say like this
There are many things you don't understand even though you feel you know them.
I hope this helps.