As the name suggests, a single repository (on github lets say) that holds all your frontend, backend, devops code.

Few repos that use monorepos are -

1. https://github.com/code100x/daily-code

1. https://github.com/calcom/cal.com

Do you need to know them very well as a full stack engineer

Not exactly. Most of the times they are setup in the project already by the dev tools guy and you just need to follow the right practises

Good to know how to set one up from scratch though