Learner's definition of ERR
to make a mistake
I may have erred in my calculations.
The court erred in refusing to allow bail.
To err is human. [=it is normal for people to make mistakes]
err on the side of
◊ To err on the side of something, such as caution, is to use or show more of it than may be needed so that you can be sure that you are using or showing enough of it.