When someone mislays something, they put it in the wrong place and then can't find it.
Mislays are usually frustrating because you have to look for something that you should be able to find easily.
Examples:
She mislays her keys all the time.
He mislays his glasses and then can't see.
I mislay my toys and then I can't play with them.
The cat mislays its favorite toy under the bed.
He mislays his homework and has to look for it for hours.
She mislays her phone and then can't call anyone.