Jack refers to a connector that is used to patch signals between modules. Jacks are typically found on the front panel of a modular synthesizer module and can come in a variety of sizes, such as 1/8-inch (3.5mm) or 1/4-inch.
There are two main types of jacks used in modular synthesizers: input jacks and output jacks. Input jacks are used to receive signals from other modules or external sources, while output jacks are used to send signals to other modules or external destinations.
Jacks are often color-coded to indicate their function, with input jacks typically being colored blue or white and output jacks being colored red. This color-coding system helps to make it easier to identify and patch signals between modules.