[Nyororin]

It says -

"Shiori -

I`m at Yui`s house. Bring your bag with you and come to Yui`s.

-Mom"

It looks like the mother was at a friend`s house (likely a neighbor) and wanted her daughter to come over there too when she got home from school. I imagine this note would have been left on the door for the little girl to see when she got home. As Japanese kids have a tendency to just shed their school bags when they reach the front door, I am guessing the mother wanted her to bring it with her (probably to do homework).