I want to upload my simple package on CTAN, but I don't know what I should enter in the "Short description for announcement" field on the CTAN upload page?
Ultimately, this is up to you as the developer. Normally, I would suggest that for a new package you put sufficient detail for someone to understand what it does, much like an abstract.
fooprovides support for [some specialist area]. It makes available [some interfaces] which are useful because ...
For updates, I favour having a simple bullet list of 'new' or 'fixed' items, but this depends on whether you have a small change or a major one. For very small changes, I tend not to announce at all.