When considering a liquid at rest, the pressure at any point within the liquid depends on the weight of the liquid column above it. This is known as hydrostatic pressure. At the surface of the liquid, there is no liquid column above it, so the pressure is solely determined by the external factors acting on it.
In most cases, the liquid is exposed to the Earth's atmosphere, which exerts pressure on the liquid's surface. This pressure is commonly referred to as atmospheric pressure. Atmospheric pressure is caused by the weight of the air column above a given point on the Earth's surface.
Since the liquid is in contact with the atmosphere, the pressure acting on the surface of the liquid will be equal to the atmospheric pressure. This assumes that the liquid is open to the atmosphere or that there is no other external pressure acting on it. If the liquid is enclosed or subjected to other pressures, the pressure at the surface would differ from atmospheric pressure.
The assumption of equal pressure at the liquid surface and atmospheric pressure is commonly made in fluid mechanics and hydrostatics calculations for open systems. It simplifies the analysis and allows for easier calculations and comparisons of pressure differences within the liquid.