Oppression can be defined as subjugation achieved through social means that affects whole categories of people. The term refers to the systemic weight of prejudice and discrimination on folks who are marginalized by it.
Conversations about oppression can get pretty complicated because of the way various oppressions and privileges interact in our society and our culture. Oppression is a systemic issue, meaning that it’s built into the systems and social structures of our society. Someone isn’t “oppressed” because their feelings are hurt, they’re oppressed, because society in some way, prevents them from having the same rights and privileges as anyone else.
An example of this would be schools who refuse to use a transgender child’s correct pronouns or name. While cis (anyone who is the gender they were assigned at birth) folk often take the privilege of being gendered correctly for granted, it can be pretty devastating for a trans person to not have those same rights.