Signal basics
What a phone is actually listening for
A mobile phone stays in touch by exchanging radio with a cell site. Phone signal is the quality of that exchange at the place you are standing. It is not a property of the room's name, and it is not a promise about download speed.
Distance sets the starting point
Radio energy spreads as it travels. Farther from a site, less of it arrives. Hills, trees, and other buildings add loss on top of distance. Two addresses on the same street can differ if one sits in a dip and the other faces the site across open ground.
Bars are a summary
The icon on the status row turns a measurement into a few steps. Different phones choose different steps. Use bars to notice a change. Use a dBm reading, when the phone shows one in its ordinary field-test or status screen, when you want a number you can compare.
Strength is not speed
A strong link can still be shared by many people. Voice may work while pages wait. A weak link can make both fail. Separate those cases before you decide what changed.