The Map display is a "track-up" display that provides information about your aircraft's programmed FMS route, status and lateral position, surrounding aircraft, and navigational aids.  The Map display has several features, most of which you can customize.  The check-boxes at the top of the Map display add different information to your display, and the buttons on the bottom of the Map display change the range of view of your Map display.  In the above image, the Map display is set to a viewing range of 320 nm, and the "TRFC" (traffic) box is checked, which displays surrounding aircraft and their flight levels.

The dashed magenta line with the arrowhead point is your aircraft's indicated heading.  This is determined by the setting of your MCP.  Notice the compass arc on the Map display, which also gives you a sense of what your heading is.  The solid magenta line is your programmed FMS route, which displays the different waypoints along your route.  In the image above, the Map display is set to a viewing range of 160 nm, and has the "TRFC" and "IDS" (identifications) boxes checked, which display surrounding traffic and their flight levels, in addition to their callsigns.

A note about TRK vs. HDG:  "TRK" is referencing the ground track of your aircraft.  It shows where your aircraft will be in space (laterally).  This is accomplished externally (visually) by rotating the display along the compass arc in order for the heading of the ground track of your aircraft to point straight ahead.  Notice here, the current heading displayed in your Map display is slightly different than your dashed magenta line (indicated heading).  Internally, your aircraft's ground track calculation is very complicated, involving your aircraft's data sensors (instruments), the FMS, and the Flight Control System (FCS).  The heading necessary to align the ground track of your aircraft with the ground track of your programmed FMS route is also computed by the FMS, and is used when your aircraft's MCP is in LNAV.  If your aircraft's MCP is in Heading Select, the Map display may change from TRK to HDG.  In this mode, the FMS is no longer comparing your ground track to your filed route, and the compass arc on your Map display will not rotate.  Because of this, your aircraft's indicated heading (the dashed magenta line) will point straight ahead. 

Here the Map display is set to a viewing range of 80 nm, and has the "NAV" (navigation aides) box checked, which displays the surrounding VORs.  In reference to the programmed FMS route line, the magenta-colored waypoint is the nearest downstream waypoint, and the other waypoints are displayed in white.  Another permanent feature of the Map display is the indication of the Free Flight / ATC boundary, which implies that all aircraft inside this boundary may not be in Free Flight.  This is presented on your Map display by a cyan-colored circle (labeled "FF/ATC") on your programmed FMS route.  This is seen in the image above next to the BAMBE waypoint.

The above image shows the Map display set at a viewing range of 40 nm, and with the "WYPT" (waypoint) button checked, which displays all surrounding waypoints, not just the ones on your programmed FMS route line.

Here the Map display is again set to a viewing range of 40 nm, and has the "RT" (route) box checked, which displays the crossing restrictions for each of the waypoints on your programmed FMS route.  Note that not some waypoints do not have a hard crossing restriction.  For these waypoints, the Map display will still show a crossing restriction, but it will only be an estimation of what your aircraft will be at when it reaches that waypoint.  This estimation is calculated by your aircraft's FMS. 

In the image above, the Map display is set to a viewing range of 20 nm, as the aircraft is approaching the runway.  This Map display also has the "TRFC" (traffic) box checked, displaying the surrounding aircraft.

Here the Map display is set to a viewing range of 10 nm, as the aircraft approaches the runway.  This Map display also has the "TRFC" (traffic) box checked, displaying the surrounding aircraft.