What are Shin "Splints"/"Splits" and what are some symptoms i should watch for?

A few years ago my first Steady job was security in which i walked a lot on concrete for 8-10 hours daily. after a while i invested in some very good police style duty boots which mitigated that for a time. the best way to surmise the way my shins felt is the feeling you get after your first skiing lesson. the front of the shin hurts a small amount but if you think of it as a hexagon the front left and front right segments hurt when i walk and ache when im not. Recently its been acting up, and i cant think of any reason why apart from daily walking and normal excersise. i have a friend who has what she calls either shin splints or shin splits, she can be hard to understand sometimes as her first language is not English. I first noticed it acting up again a few years ago when i was undergoing Police academy training. it would be the same every day and got to a point where i would have to sit out PT every once in a blue moon. after i graduated and wasnt as active as id like, it seemed to go away. now i do have flat feet but ive never really asked anyone what all these terms mean. So thats the story, hope somebody out there has some insights.