A radio frequency band near 750 MHz. In the US, it's owned exclusively by Verizon and used for 4G LTE service.
Band 13 is just one part (Block C) of the Upper 700 MHz band.
See: Upper 700 band
Like most radio frequency bands used for phones, band 13 is a paired band, meaning it's split into two parts: one for phones to transmit to towers, and the other for towers to transmit to phones. Phones transmit at 746 - 757 MHz, while towers transmit at 776 - 787 MHz.