I have an electrical Question. I am familiar with polarity But I don't know what this <-> symbol means

Answer 1

A plus sign (+) denotes the positive connection to a DC power source. A minus sign (-) denotes the negative connection to a DC power source. It can also denote a negative voltage, a voltage less than zero volts relative to the power source.

In the case of a circuit that uses a negative voltage, there will usually be a reference to "ground" where ground is not necessarily connected to the earth (the original definition of the "ground" term) but is a zero-voltage point against which all other voltages are referenced.

Answer 2

I wonder if this is a polarity symbol used in DC power supplies. Defines whether + is in the inside or the outside.

See: http://www.accesscomms.com.au/reference/polarity.htm