To “bear in mind” is to be considerate and mindful of a piece of information.
The phrase is often given as an instruction. For example:
When you’re organizing her bachelorette party, bear in mind that she doesn’t really like drinking.
Here, the speaker of the sentence is asking a person to remember the fact that the “her” being spoken of doesn’t like drinking, and to consider that fact in all of the plans they make.
