One of the purpose of the Sprint Retrospective is to: "inspect how the last Sprint went with regards to people, relationships, process, and tools".
What mean relationships?
The other three are pretty self explanatory: people are the individuals involved in the project and how they conducted the work, processes are how the work was done, and tools are the things necessary to achieve the work. Relationships refers to the connections between these things.
With the people-orientation that the agile methods have, that it tends to refer to the relationships between stakeholders (within the Development Team, Development Team/Product Owner, Development Team/Scrum Master, Product Owner/stakeholders, Scrum Master/Product Owner). However, it could also refer to the relationships between the tools and processes (if the tools are helping or hindering the team's workflow), the processes and the people (if the process preventing people from being their most productive), or the people and the tools (if the people have the knowledge and skills to use the tools effectively).
What this is suggesting is that you look at all factors of the sprint to find things that are going well (so that you can continue to do them or do them on other projects) and to find things that aren't going well (so you can correct them), even if that means looking beyond a single unit.
In some tasks, it's a good idea to work in pairs. So, at this point, in a retrospective, it can be useful to talk about how things worked in a pair. For instance, you can talk about any coordination problems between the colleagues, etc.
[Edited] For example:
A Product Owner -Named Y- defines his User Stories (US) with X AT's. US is revised and estimated but, when a programmer is working in this US PO sometimes adds news AT's and the US grows (more complex, more time...).
Well, in the retrospective we must talk about the problem when tech team works with PO Y because tech team has problems for completing Y's tasks caused by his bad praxis.