My Mac won't up date to Ventura. What does this mean?

Apple typically provides security updates for the two previous versions of the OS (and sometimes three). So, since your mac can run Monterey, one may expect security updates for at least two more years.

And it’s not as if your mac will stop working in two years, either.

The age of your mac will determine if it can be upgraded or not.

Go to (Apple menu)->About this Mac.

Post a screenshot, not including the serial number.

Or tell us what the model and year are: it may say something like MacBook Air (13", mid 2015).

The model and year will let us and you know if it is supported in Ventura.

You can also check if your Mac is compatible here…

macOS Ventura is compatible with these computers

FWIW, there was no Late 2015 MacBook Air. Assuming you have an Early 2015 MacBook Air and you are running Monterey on it, it will be supported for at least 2 years.

you're welcome.