As we know FETs have three terminals: source, gate and drain.
Open source means that the source terminal is floating: it is no connected to either the GND or the VDD of the power supply. In this case you must use a pull up or pull down resistor depending on the connection.
Open drain means that the drain is floating. You must use a pull up or pull down resistor depending on your connection.
Integrated Circuits may be addressed as open drain or open source, it simply refers to the output FET used in the IC.
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en.m.wikipedia.org/wiki/Open_collector