It is a device that works as an atmospheric sensor, designed by Tinker Tom. They are used to detect any anomalies in the air when placed on vantage points, along with housing cameras and low tech surveillance gear in order to monitor surrounding areas.
↑Sole Survivor: "Yeah, that's really great, Tom." Tinker Tom: "Where was I... Oh, yeah. So my sensor, I call her MILA, needs to be up high. Real high. Dez forced me to put some cameras and other low tech surveillance crap on it to watch the Freedom Trail." Tinker Tom's dialogue