To understand Paksha, you first need to understand Masa clearly. Paksha in the Hindu Panchanga refers to the two halves of the Moon's monthly cycle. In other words, we divide a Masa into two parts, and each part is called a Paksha. Each Paksha is marked by a change in the Moon's appearance, known as waxing and waning.
In essence, Paksha is a way of measuring time by observing the Moon's cycle of growing (waxing) and reducing (waning) in brightness.