penser is to think in general, whereas réfléchir is a kind of deeper thinking, like to reflect, to ponder. But in many situations, they are both translated as "to think".

It is often best to look them up in an online dictionary with sample sentences. One can get a good feeling for the word that way. Here is one such dictionary, but there are many:

https://www.linguee.com/french-english/translation/r%C3%A9fl%C3%A9chir.html