airlines usually want the flexibility to move seats around or remove them, so the seats are attached to rails underneath the floor which run along the aircraft fuselage. If the airline wants to reconfigure the seating, this is a minor operation.
For passenger safety, airline seats are equipped with seatbelts, and there is a "Fasten Seatbelts" sign above each seat which is lit up when passengers are expected to remain seated with the seatbelt fastened. This is during taxiing, take-off and landing, although turbulence may also prompt the captain to turn on this sign.