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Callsigns are a group of letters and numbers for use on the radio net that indicate a person, vehicle or unit on the radio net. The Senior station on a net is called 'Control' and has the callsign 0 ('Zero'). The reson for being called 'zero' is that despite being called the 'control' station it is the one station on the net that has time to make brews and get their admin done, so are invariably anything but 'in control', so callsign 0 has 'zero' clue about what's going on.
Most of the callsign remains the same every day, so it's really quite simple that 10A today will still be 10A tomorrow. But on the modern battlefield there are many different units that may be sharing the same net, which may mean that there are two different 10A callsigns on the same net at the same time. To stop things getting too confusing each callsign is made unique with the addition of a letter at the front of the callsign, called the 'callsign indicator'.
In the good old days the callsign indicator was set depending what troops were using it. Eg. 'Tango' was the callsign indicator for Armour giving T10A, 'India' was the callsign indicator for Infantry giving I10A etc. This made life quite simple on the net as officers could move whole groups of assets around the battlefield in simple orders like "All Tango units move now!" to move all the tanks on the net at the same time.
Now things aren't so simple as the callsign indicator changes daily to confuse the enemy. This is achieved at the expense of confusing ourselves just as much, and may be the reason that many radio operators have bald patches as a result of pulling their hair out, trying to get officers to change callsign every day. When they do grasp the concept of the callsign changing daily you have to spend hours trying to convince them that "I know you are a clever boy and you know your alphabet, but just because we were B10A yesterday doesn't mean we are C10A today!"
Maybe the films had it right after all.... remember, "Broad Sword calling Danny Boy, Broad Sword calling Danny Boy"?