+ and - meaning in Tx+ and Tx- in cable pairs?
The positive and negative connectors in the cabling mentioned, form a balanced pair for signals. They typically carry identical signals of opposite polarity, i.e. a HIGH bit may be +5 Volts on the + conductor, and -5 Volts on the - conductor. The corresponding LOW bit would then be -5 Volts on the + conductor, and +5 Volts on the - conductor.

Having such signals of opposite polarity in a twisted pair of wires helps in reducing susceptibility to common-mode noise / EMI. Because the voltage difference between the two conductors for any valid bit is twice the actual signaling voltage, this has the effect of doubling the strength and hence the detection of the signal at the receiving end.

Specific to the cabling / protocol involved, various signaling schemes may be used for transmitting the actual bits down the line, from the simple Gray Code, to more complex non-return-to-zero (NRZ) or other coding.