A "pressure gauge on an HTP pump" refers to a measuring device attached to a High-Pressure (HTP) pump that indicates the pressure of the fluid being pumped, allowing operators to monitor and control the exact pressure level within the system, crucial for applications like agricultural spraying where precise pressure is needed for effective distribution of liquids like pesticides; essentially, it's a gauge that tells you how much pressure is being generated by the HTP pump at any given time.