Blood pressure is the force of blood against the walls of the arteries, recorded as two numbers: the pressure when the heart beats and the pressure between beats.
The first number is systolic pressure and the second is diastolic pressure. A reading is written as one over the other, for example 120/80, measured in millimetres of mercury.
It matters on this site for one reason. ADHD stimulant medication can raise blood pressure and heart rate, so both are checked before medication starts and again at reviews. NICE guidance on ADHD includes this monitoring.
If you already know your blood pressure is high, say so at your assessment. It affects which medication is considered and how closely it is monitored.