Data transmitted from the space station (sensor values, commands, other information...) and received on the ground is called telemetry. The purpose of the Mimic program is to retrieve the publicly accessible telemetry from the ISS and display it in an informative manner and also to utilize some of it to transmit to arduinos to run motors and match the physical orientation of the moving ISS joints.
Telemetry Signal Status
Signal Acquired aka AOS (Acquisition of Signal)
The "Signal Acquired" status indicator means that the program has a live connection with the NASA mission control public data adapters transmitting the actual data from the ISS.
Signal Lost aka LOS (Loss of Signal)
The ISS transmits telemetry by pointing moving antenna dishes (called the Space to Ground Antenna or SGANT and the S-band Antenna Support Assembly or SASA) at a Tracking and Data Relay Satellite (TDRS - pronounced Tee-Dress) which then relay the data to a ground station which forwards the data to NASA. There are three groups of TDRS satellites, that are in geostationary orbits over the Indian Ocean (TDRS-Z), Pacific Ocean (TDRS-West), and Atlantic Ocean (TDRS-East).
The three TDRS groups have overlapping coverage area so there can be continuous data transmission from spacecraft (ideally). However, because there is only one satellite in the TDRS-Z group it is only utilized during critical operations (spacecraft docking to the ISS or during a spacewalk...), so during normal operations the ISS will always lose signal over the Indian Ocean (known as the Zone of Exclusion (ZOE)).
Even with all three TDRS groups operating, the ISS will always lose signal during a "handover". While the ISS travels along its orbit, it passes under the TDRS satellites which remain roughly stationary over the same point on Earth, so once the ISS is getting close to the end of one TDRS group's coverage, the moving antennas will have to stop pointing at one TDRS satellite and will have to swivel all the way forward to acquire the next TDRS satellite along the ISS orbit. During these swivels (handovers), the antenna will be pointed between satellites and can't transmit information to the ground.
So in summary, Signal Losses will occur primarily: Every time the ISS passes over the Indian Ocean (for up to 15 minutes until the ISS gets into the coverage zone of the TDRS West satellites), or every time the ISS antennas swivel to point at the next satellite group after reaching the end of the previous satellite coverage (this LOS should only last for no longer than a few minutes).
Occasionally the signal will be lost for an extended period of time (a few hours) for other reasons. not aliens - promise ;)
This event has nothing to do with ISS data transmission. Occasionally something goes wrong with the public data adapters in Mission Control and the data stops updating but is not due to an LOS from the ISS (this only affects the public telemetry output, Mission Control is still receiving data from the ISS). If the timestamp for the last data value received lags behind the current timestamp by a certain amount (and an LOS status is not received) the data will be marked as stale. The stale data state can last anywhere from a few minutes to a couple of weeks. We have no control over when this happens (luckily it does not happen that often).
Upcoming Loss of Signal
The program will attempt to predict when a Loss of Signal with the ISS will occur, based on longitude and antenna gimbal angles. This icon indicates that the signal will be lost momentarily.