Medial tibial stress syndrome (MTSS) is a coming overload injury that we primarily see in our sporting patients - runners most of all. It is the more accurate term for what some people mean when they say "shin splints" - a term we avoid in clinic as it's too vague and can mean pain in different places around the shin so isn't helpful.
The pain with MTSS is experienced around the middle to lower third of the anterior-medial aspect of the tibia (the front-inner part of your shin essentially). The pain usually comes on with running (or very brisk walking) and, over weeks and months, if symptoms worsen it will tend to come on faster and faster one activity begins. It is often painful enough that a runner will stop activity - at which point the symptoms will generally subside very soon afterwards.
Another condition that many (including us) believe is on the same spectrum as MTSS is a tibial stress fracture - the onset and nature of the symptoms can be quite similar but may linger for longer and the site of pain is usually easier to pinpoint - It is VERY important you see the experts (we like to think that's us!) to have this problem investigate thoroughly as it is a challenging condition to treat.
More important still - if you are experiencing an extraordinary amount of pain in the front of the shin brought on by exercise or a blow to the shin (such as a bad tackle in football that continues when you are at rest, there is the possibility we are dealing with a condition called compartment syndrome - this is a medical emergency and you should seek medical attention immediately, even if only to have this ruled out.