I got given a Garmin Vivofit with a heart rate monitor chest strap from work today.

Anybody know anything about it? Is it any good, useful for mountain biking my style seems to be explosive start and stopping.
Will I need a road bike for it?
Is it #enduro?
Can’t download the app for it, my phone is a relic of times gone by.

What are you supposed to do with them?
I know nothing.