ground
Ground is the agreed zero point of a circuit, the reference level that every other voltage is measured from. Remember that voltage is always a difference between two points; ground is the point we all agree to call 0 V, so that saying this pin is at 3 V has a clear meaning, namely 3 V above ground. It is like sea level for altitude: mountains and valleys only have heights because we agreed where zero is.
In a battery circuit, ground is usually just the battery's minus terminal, drawn with its own symbol and connected to one common rail that wires weave back to. It is not necessarily connected to the actual earth; that is a separate idea. Ground in small electronics mostly means common: the shared return path that all currents flow back through and the point all measurements are taken against. When you measure voltage with a meter, you put the black probe on ground and the red probe on the point of interest.
A solid common ground is one of the most underrated design decisions. Many beginner circuits fail simply because two parts do not share a ground, so there is no complete loop and the voltages float meaninglessly. In careful designs, where and how grounds join matters a lot: shared ground wires carry shared currents, and a sloppy ground can inject noise. For now: pick one node, call it ground, and refer everything to it.
To read a sensor's output you connect the meter's black lead to the circuit's ground and the red lead to the sensor pin; without the shared ground reference the reading is meaningless.
Every voltage is relative to ground, so measurements need a common zero.
Ground does not always mean the earth; in most hobby circuits it just means the common 0 V reference, and earth ground (the third mains pin) is a distinct safety concept, so do not assume they are the same.