You can use it to describe something's permanent state or a temporary condition: for example, if you say det är en lugn gata (it's a calm/quiet street), it means that it is street where there's never much noise or traffic, but if you say gatan var lugn (the street was quiet), that just refers to a particular point in time. If you say han var lugn (he was calm), this can mean 'calm', 'unperturbed', 'unworried', 'unbothered' or 'relaxed', and you'd usually be describing the person's state in reaction to a specific incident. For example: du måste lugna ner hunden (you need to calm the dog down); när han lugnar ner sig ska han kunna sova (when he calms down he'll be able to sleep).