It's all the systemic and cultural things that propagate gender-based hierarchies, usually with white men at the top. It's a concept used frequently in feminism. https://gender.study/gender-based-violence/understanding-patriarchy-feminist-perspectives/ has a short intro (you don't have to read the critiques part if it's not interesting) that covers some different aspects. There's a lot that can be placed under the umbrella like reproductive rights, gender norms, domestic violence, and toxic masculinity. Even though the patriarchy is theoretically advantageous to those placed at the top, it harms everybody.