A system is composed of parts these relate to each other in a structure or through behaviors. A structure can be a physical space, or a time space. Dates in a calendar relate to each other in time. Underwear in a drawer relate to each other spatially in containers. A behavior is a process that transforms input to output. When putting socks and bras into your underwear drawer (input) you might put all the socks in one basket and all the bras in another (output), the behavior sorts underwear according to type.
Most systems interact with their environment in some way, they are open. Scientists often like to pretend that their systems are closed, that no exchange happens with the environment. This is usually a gross simplification that makes observations easier to explain. For non-scientists looking at their systems it is important to remember that systems live in a context. Looking at underwear again; my system works in the context of my life. I share my space with one non-bra wearing person, which in this case means the bra process can process any bra into my bra-drawer. In another household there might be more bra-ed people and the process needs to include a behavior to determine which bra-drawer a bra goes to.
Anything outside the system, the surroundings, can be ignored. That my neighbor also wears bras and processes them while doing laundry is irrelevant, so is the cute bra with polka dots in the store. You don’t need to account for it in your process.
What is important to remember about systems, in a personal context, is that we all have them. Even if you can’t see them, or they aren’t doing the things you want them to do. Next steps are making the systems visible and then changing them to better do what you want.