Philips Hue lights will work without a bridge for the basic functions provided by the dimmer (On/Off, Brightness & selecting from 5 predefined white light "recipes" (bright white, ice white, warm white, dimmed warm white & nightlight)
You can also "pair" additional dimmers to the same lamp (without a bridge) or "pair" an additional light/bulb to a single dimmer (without a bridge).
There are videos on the Philips Hue channel for how to do this.
I have 1 lamp with 2 dimmers (one at each end of the stairs) & another dimmer which controls a whole room of lights (3 separate lamps) without a bridge.
You can also use the Bluetooth functionality (on the newer devices) to control them via the HueApp (or other 3rd party ZigBee/BT apps) without a bridge but its not really practical when several people live in the same place.
The bridge provides extra functionality like timed routines, remote access & location based functions (like lights off if your phone is not in the house).
However, I recommend to read about the drawbacks of the Hue system before going all in.
e.g. there's no way to backup or save you hub configuration (so if it breaks you have to add all the lights again 1-by-1), the switches, dimmers, buttons all have different (non-configurable) behaviour & Philips provides very basic support for "smart home" functionality like IFTTT or Alexa/Google.
The "BT" in the name stands for Bluetooth. This means that the lamp can also be controlled via the app if you do not have a Hue Bridge. For this, the "Hue BT" app must be installed instead of the "Hue" app (bridge). The range of functions is not as large as with the bridge, but still decent. I would definitely recommend installing it, as it allows the colour temperature to be changed, which has a strong effect on the light pattern (from bluish to white to yellowish).
