@shasta technically speaking it's organized violent persecution of an ethnic group. Historically has a more specific meaning as it originates in Russia.

When the Russian Empire expanded and in the 19th century began to colonize /occupy territory that contained significant ethnic Jewish populations, they undertook a lot of rioting to lynch/massacre/expell them. These were known as "pogroms".

In modern English it means this kind of systematic attack against any ethnic group. It is also sometimes used informally/metaphorically for other things e.g. political or corporate "pogroms" against a particular group or class of people.