Pull up....
On signal lines that can be connected to more than one device (in this case I2C's SCK and SDA), the signals aren't powered directly by any of the chips, but by power and the pullup resistors. The various devices can only apply a ground to the signal line. If no devices on the bus are applying a ground, the line stays high (logic 1, at near power supply voltage). If there are multiple devices on the bus, and any one of them applies a ground, the entire signal line drops to ground (logic 0).
If you had each chip holding the signal lines high, and one of them tried to make the signal line a low (logic 0), the other chips would either overpower it, or you'd end up with a lot of smoke in the room
Same thing goes for a pull down, but the other way in reverse.
Most of the time, the terms 'pullup' and 'pulldown' aren't so complicated. A pullup just means a high-ish value (4.7K, 10K, etc) resistor between Pin X (or whatever) and the positive supply rail. Conversely for the pull down, a resistor between Pin X and the ground rail.
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