"Fill someone in" is a common phrase that means to provide information or details to someone. However, there are many different synonyms that can be used to convey a similar message. Some examples include "bring someone up to speed," "provide an update," "educate someone," or "keep someone informed." Other synonyms could be "give someone the low-down," "brief someone," "enlighten someone," or "inform someone thoroughly." Each of these phrases can be used to convey the same general idea - that you are providing important information to someone else in a clear and concise manner.