What Do Signal Bars on a Cell Phone REALLY Mean?
In all honesty, the bars on a cellphone do not really mean anything. They are an aesthetic representation of the signal strength. The signal strength is measured by decibels. Depending upon the make and the model of the phone, there can be quite a bit of fluctuation between the bars on one phone to another phone.
For example, if an algorithm of your phone says that for every 10 db you will see a bar and your friend's phone manufacturer has set it such that for every 15 db a bar will appear, their signal will always appear to be weaker even though they may be receiving more dbs (40 dbs which would be 2 bars as opposed to 30 which would show 3 bars). Bars could be compared to a gas gauge where they give you a general idea of what the signal strength is, but not the actual strength.
Therefore, it is more effective to measure signal strength in decibels to determine the best place to find a signal or use a booster. Typical Android phones with have this function in their "settings". Typical iPhones can access this information by putting phone in test mode by typing *3001#12345#* followed by "Call" in the phone's number key pad - The Decibels appear in top left corner or by sliding down from the top edge. -50 dB would mean maximum signal and -110 dB would mean no signal. Your range would be somewhere in the middle of those two numbers.
Additionally, there are several apps available which will display your DBs rather than having to rely upon the bars.