On the latch
Definition 1 of 1
Pro Tip 1/3Casual Security
The phrase implies a casual or relaxed attitude towards security, as the door isn't fully secured. 
I'll be in the garden, just leave the door on the latch so I can get back in.
Pro Tip 2/3Common Expression
Often used in everyday English to indicate that entering is easy and no special effort is needed. 
Just come in when you arrive; the door's on the latch.
Pro Tip 3/3British Usage
Primarily used in the UK, so understanding this can help in comprehending British English contexts. 
We keep the back door on the latch during the day for the kids to come and go.