Here is Wikipedia's definition of exclusionary zoning:
Exclusionary zoning is the use of zoning ordinances to exclude certain types of land uses from a given community, especially to regulate racial and economic diversity. In the United States, exclusionary zoning ordinances are standard in almost all communities. Exclusionary zoning was introduced in the early 1900s, typically to prevent racial and ethnic minorities from moving into middle- and upper-class neighborhoods. Municipalities use zoning to limit the supply of available housing units, such as by prohibiting multi-family residential dwellings or setting minimum lot size requirements.
This is a common way to think about it. Prohibiting multi-family residential is a way to try and keep renters away. And mandating minimum lot sizes is a way to ensure that lots don't get subdivided and that nobody builds homes of, you know, lesser value.
It's more or less a way of setting a minimum bar, which is why the term exclusionary zoning is used. If you don't meet this minimum bar, you are excluded.