Basis of a topology

A basis of a topology is a collection of open sets from which all other open sets in the topology can be constructed through unions. Think of it as a foundational toolkit: these basic open sets are like building blocks. Any open set in the space can be formed by combining (taking unions of) these blocks. The basis simplifies understanding the topology because instead of describing every open set individually, you only need to specify this smaller, manageable collection that generates all others.