It can be very confusing when laptops display the Bluetooth symbol on them, as it gives the impression that the laptop has Bluetooth built-in. Unfortunately, this is not always the case.
My laptop displays a Bluetooth button, yet when I press it, it says I have no Bluetooth device installed. This is because there are several variations of models built, and they are fitted into the same casing, but all of them will vary in specs. Eg. there may be another variation of your model that looks identical but has a different processor and a different amount of RAM.
To check if your laptop has built-in Bluetooth, either check your manual or go into your Device Manager to see if you have any Bluetooth hardware installed. If you don't, you will need to buy a Bluetooth dongle. These are very neat and tiny, and can be bought on eBay for around a fiver (including P&P).
If you don't know what Bluetooth is - as suggested by your question - it's simply a form of connectivity. Most mobile phones have Bluetooth these days, so if you had some music or photos on your laptop that you wanted to transfer to your phone, you could connect them via Bluetooth instead of messing around with cables. In these situations, Bluetooth can be very useful. However, you do need to be in fairly close proximity to connect (around 50 metres).