Everywhere I have lived has its own variety of humour, which may, or may not, translate, and can sometimes be misunderstood in ways that are rather unfortunate.
Glasgow humour is no different. A lot of it seems to depend on puns, and this sign, in a cafe now shut for the duration, is a superb example of it.
Shut happens - and for now the nation is pretty much shut. But open will come again, if not soon, at least one day. This is hope. This is resilience. This is what faith is about... shut happens, but we choose to trust that open will happen too!