The little white Windows looking icon that has probably appeared on the right side of the task bar that that is at the bottom of your screen is for Windows 10.
If Microsoft has determined that your machine qualifies to be upgraded to Windows 10 then the icon appears. The qualifications they are looking at are whether you are running Windows 7, 8 or 8.1 and does your computer have the proper equipment to run Windows 10.
This button if you select it will allow you to get Windows 10 upgrade free after July 29 as they issue the upgrades. Microsoft has said they will not send out all the upgrades on day one (July 29) but will send them out in a rolling method with different ones on different days.
This upgrade is free if you qualify (and thereby get the white Windows icon in the task bar) as long as you upgrade within the first year, basically before July 2016. If you decide to upgrade after 1 year then you will have to pay a monthly subscription fee to use Windows 10.
When I get issued my upgrade I plan to upgrade my laptop or desktop immediately and will write a column on the process at that time.