The concept of the "observable universe" refers to the portion of the entire universe that we can potentially observe from our vantage point in space and time. It is important to note that the observable universe is not the entirety of the universe itself.
The reason we speak of an observable universe is due to the finite speed of light. Light travels at a specific speed, and as a result, the information from distant regions of the universe takes time to reach us. The farther away an object is, the longer it takes for its light to reach us. This means that we can only observe objects or events that have had enough time for their light to reach us since the beginning of the universe.
Since the universe is estimated to be about 13.8 billion years old, our observable universe is limited to a sphere with a radius of approximately 13.8 billion light-years centered around us. Beyond this observable sphere, there may be more galaxies, structures, and phenomena that we are currently unable to observe because their light has not reached us yet.
It is worth noting that our understanding of the universe's size and structure has evolved over time, and scientific advancements continue to refine our knowledge. However, the concept of the observable universe remains a useful framework for understanding the limitations of our observations based on the finite speed of light.