Citizens in the US are implicitly allowed to do whatever they like, subject to laws that the government enacts. The constitution describes those areas where the government is allowed to pass laws. All other areas are off limits to the government, and left for the people to do as they like. To emphasize the point, the amendments specify certain areas that the government is extra-especially-not-allowed to create any laws about, like speech.
The extent to which this is observed today is quite dubious. There are lots of laws that the US government passes which have little to do with anything the constitution allows them to do - but they kinda hand-wave around that and gesture toward something, like the "commerce clause" or whatnot as justification.
But in theory - for any law passed - it is unconstitutional unless you can say exactly where in the constitution it is explicitly allowed.
* Having written all that, I will add that "government" above means the US Federal government, not all the other ones. State, local, have a lot of latitude to make whatever laws they want, unless a federal law specifically prohibits it.