ESXi runs in "trial mode", which is effectively a time limited Enterprise Plus license, for 60 days, after which it will expire and be unusable. You won't lose your VM, it'll just refuse to be managed until you install a license.
The trick here is to fill in the online form for a "free" license. You should have found that near the download page. You can install this license for unlimited use. It definitely is absolutely completely free, with the caveat that many of the advertised features of VMware are licensed features and will be locked out of the free version.