Software isn’t easy to produce. This is why it goes through extensive testing before being released.
However, inevitably—because coders and testers are human—there’ll be small errors (known as bugs) in the code that were missed in testing. A few of those errors will leave security holes that need to be patched; some will mean that the software doesn’t work exactly as intended. Once found, they’ll need to be fixed.
Sometimes new threats are identified, that weren’t taken into account when developing the code initially. And occasionally, updates that were intended to fix errors introduce new problems, which are identified later.
This is why software manufacturers issue new versions of their software, designed to fix errors and patch holes. These new versions are called updates, or patches.
They are not the same as upgrades. A software upgrade is a version of the software that supersedes the old version: it is intended as an improvement, offering new functionality, better speed or efficiency.