It just means the minimum amount of time between flights that an airline will require. They will try not to let you book a connecting flight 10 minutes after the first one lands; that would be an "illegal connection". Not an actual law, just airline policy.

I have the fun of going through Secondary every time I touch US soil... for me, if I have a connecting flight on the way from LHR to SFO (via Dallas, LA or Chicago) then I need to allow 3 hours for the connection.

It is very hard to book such flights, and if booked as 2 single hops I've once before missed the connection due to being in security and United argued that the second flight was not a connection and so they didn't have to put me on the next flight. My baggage of course had gone ahead.

Expanding a bit: the more usual term is MCT -- "minimum connection time". There are actually multiple categories of MCTs:

* MCT D-D: Minimum connection time, from one domestic flight to another domestic flight.

* MCT D-I: Minimum connection time, from a domestic flight to an international flight.

* MCT I-D: Minimum connection time, from an international flight to a domestic flight.

* MCT I-I: Minimum connection time, from one international flight to another international flight.

These are set per airline per airport, and in many cases there are more fine-grained MCTs for flights depending on origin/destination or categorized by flight number. The airline will not let you purchase tickets for an itinerary which violates the relevant MCT, and if delays cause you to dip below the MCT you're possibly going to be rebooked (though no guarantee of it).

For completeness' sake, here's what the information looks like in raw form (this is the MCT for American Airlines at its hub in Charlotte):

    STANDARD.D/D...D/I...I/D...I/I.
    ONLINE    .40  1.00  1.00  1.00
    OFFLINE   .40  1.00  1.00  1.00
    ** OR * ARE ALL
    AA-AA ID   .30 FLT    1 - 2754 - FLT 2755 - 6099 
      EQP ALL - E70 PUERTO RICO - ALL   
    AA-AA ID   .30 FLT    1 - 2754 - FLT 2755 - 6099 
      EQP ALL - E70 VIRGIN ISLAND US - ALL   
    AA-AA ID   .30 FLT    1 - 2754 - FLT 2755 - 6099 
      EQP ALL - E75 PUERTO RICO - ALL   
    AA-AA ID   .30 FLT    1 - 2754 - FLT 2755 - 6099 
      EQP ALL - E75 VIRGIN ISLAND US - ALL   
    AA-AA ID SUP   FLT    1 - 9099 - FLT    1 - 9099 
      CANADA - CANADA  
    AA-AA ID SUP   FLT    1 - 9099 - FLT    1 - 9099 
      MEXICO - MEXICO  
    AA-AA ID   .50 FLT    1 - 6099 - FLT    1 - 6099 
      IRELAND - ALL   
    AA-AA ID   .30 FLT    1 - 2754 - FLT    1 - 2754 
      PUERTO RICO - ALL   
    AA-AA ID   .35 FLT    1 - 2754 - FLT 2755 - 6099 
      PUERTO RICO - ALL   
    AA-AA ID   .30 FLT    1 - 2754 - FLT    1 - 2754 
      VIRGIN ISLAND US - ALL   
    AA-AA ID   .35 FLT    1 - 2754 - FLT 2755 - 6099 
      VIRGIN ISLAND US - ALL   
    AA-AA ID  1.15 FLT    1 - 2754 - FLT    1 - 6099 
       GROUP3 - ALL   
    AA-AA ID  1.00 FLT    1 - 6099 - FLT    1 - 6099 
    **-AA ID  1.15 ALL  - FLT    1 - 6099 
    **-AA ID SUP   ALL  - FLT 6100 - 9099 
    AA-** ID  1.15 FLT    1 - 6099 - ALL  
    AA-** ID SUP   FLT 6100 - 9099 - ALL  
    USE T*GRPN FOR CITIES IN A GROUP.  WHERE N IS THE GRP NUMBER.

The first few lines give the general rule ("ONLINE" means both flights on the same airline, "OFFLINE" means connecting to a flight of a different airline), and after that come exceptions. For example:

    AA-AA ID SUP   FLT    1 - 9099 - FLT    1 - 9099 
      CANADA - CANADA  

"AA-AA" means from American Airlines to American Airlines. "SUP" means "suppressed": the connection is invalid. "FLT" gives a block of flight numbers the exception applies to (1 through 9099). "CANADA - CANADA" gives the origin of the inbound and destination of the outbound flights. So this is a fancy way of saying American forbids booking a flight from somewhere in Canada, to somewhere in Canada, connecting in Charlotte, on a flight numbered 1 though 9099, regardless of the amount of time of the connection. Other codes like "EQP" refer to the type of aircraft ("N" for narrowbody, "W" for widebody, "ALL" for all types).

You can spot other fun things in there, like the shorter time to connect on a flight from Ireland to a domestic flight; AA flies to Charlotte from Dublin, and Dublin has US Customs preclearance facilities, so passengers arriving from Dublin don't have to clear customs in Charlotte and can make a tighter connection.