I'm an architect in RI and MA, so take my advice with a grain of salt & then research your area.
Zoning is LOCAL law that applies to property, it regulates the USE of the property (IE: single family, multi-family, commercial, etc) and the DIMENSIONS of the property (IE: how big does the property need to be for a particular use)
Every scenario and property are different, but here's a couple of examples:
An owner calls me. They have an existing two-family home. They want to convert it to a three-family home. I look at the underlying zoning and realize it's in a one-family zone. It's highly unlikely they'll be able to do the conversion, but if they did, they'd be required to get a variance.
(Note: a variance is a process whereby you can get "relief" from one or more parts of the zoning ordinace. A typical variance would be to build closer to a setback than is allowed "by-right" in the zoning ordinance. Another typical variance would be to build on a property that is too small than allowed "by-right" in the zoning ordinance.)
Another example:
A client calls. They have a residential property that they live in. They want to have a small business in this property - say, a lawyer's office. It'll be just the resident working as a lawyer, and occasionally she will have clients come by for in-person consults. I look at the ordinance and discover that commercial use isn't allowed in the zone, but a "home occupation" is allowed by special permit. We help the client get the special permit, and then they can use part of their home as an office.
(Note: a special permit is a different type of process, very similar to a variance, where you can get special permission to do something under the ordinance.)
Generally, as investors, we want to discover new ways to make money on our properties - either adding units or building more units than allowed. This almost always involves a variance. The more you work with a zoning ordinance the more you'll understand. It's also very dependant on the local people - both the "staff" (IE: people who work for the town in the planning, building, zoning depts, etc) and on the members of the board (zoning board, planning board, etc). Some communities are very easy going with their variances, and others never grant them. Likewise, some zoning ordinances are more easy going than others.
For example, in my area, Providence RI has NO minimum property size for single-family homes. Newton MA requires something like 15,000 SF for a single-family home. Both can be excellent investment locations, but the rules are different.
I hope this helps. You can always have a discussion with the local staff, and they can help you understand their ordinances and the process.