Bluetooth is the mechanism fitness bands use to transfer their data to your mobile phone.
It is the same way hands-free phones work in cars.
(It's named after a Nordic Pirate .... who probably had bad dental hygiene ... and was designed in the days when Swedish Ericsson and Finnish Nokia were big in the mobile phone market ... but you don't need to know that to choose a fitness band
)
As for recommendations, I guess it depends what you want to use it for and how much you want to pay,
For example, are you happy to just count your steps or do you also want it to measure your swimming and cycling?
How often do you want to charge it? Many need charging daily but some can last a month ... or more if they have replaceable batteries.
Do you want to extract the data to be able to review your targets over the last month, year ... or just see what you have done today and forget about it? Many people aim for something like
an average of 10,000 steps per day which gives them flexibility to have a chilled day every so often. This is what you need that pesky pirate for.
Do you intend to wear it all the time or just when you exercise? Wearing it all the time will count those steps you forget about like when you go and put the kettle on but it will replace your watch so you need a clock on the band and you
may want a smaller band (some of the fancy ones are very chunky).
Sorry, not exactly a recommendation but I hope I have helped rather than confused.
