The way I think about Monthly Active Users (MAU) is as a rough measure of how big my community is currently. Every month, this number of logged in users visits at least once. It excludes visits from guests who are not logged in, which is good, as those visitors are not quite stepping across the threshold. It also excludes all the people with accounts who haven’t visited that month. Those not counted may be members who are unlikely ever to return, or who come occasionally, but not every month.
The way I think about Daily Active Users (DAU) is as a rough measure for how busy my community is currently. Every day, this number of logged in users visits at least once. It doesn’t distinguish between users who visit every day and users who only visit once a month, but it does tell me something about how many different users are coming to interact in some way on a given day. Again, it doesn’t account for guests who do not log in.
So then, DAU/MAU can be thought of as how busy my community is (DAU), but it in a way that automatically adjusts to its current size (MAU).
Or you can think of it the other way around. Given how big my community is (MAU), how many of those users show up on a given day (DAU)?
In any case, dividing by MAU is what helps this metric automatically adjust for the current size of your community.
Different communities will certainly have different expectations for this metric. Deciding what you think good looks like for this number certainly depends a lot of your own understanding of your community.