If you're making translations for a project that uses a Structured or Restricted permissions policy and you're a member of the appropriate translation team you can work in one of two modes:
When you're reviewing translations made by other people, or you're working in a small team, you usually want your translations to be applied automatically. This is when you'd choose Reviewer mode.
However, if you're working in a team that uses a peer review process Translator mode will be more suitable as it holds each of your translations as suggestions that should be reviewed by someone else.
There's more help with translations in our help wiki.