Map boundaries are invisible walls near the edge of every map that stops players from falling off the edge
The map boundary is considered a "wall" in the game. Just like any other obstacles, the player is able to jump very high while touching it. Most normal spells cannot penetrate the boundary and projectile/multi-projectile spells would explode when shot at the boundary. Players outside the boundary would not be able to damage players inside the boundary with normal spells (except oblivion). Certain ultimates that does not shoot projectiles/beams, like Gravitational Pull, will be able to affect players on the other side.
Out of the Boundary Glitches
There is currently one known way of getting out of the boundary - by using Shadow Sneak. (Edit!:Aurora transportation spell and reaper's transportation spell works too.)This requires the player to have void, so most players would not be able to get out of the boundary unless teleported in by another player using Oblivion. Players outside the boundary cannot walk back into the boundary. They have to teleport back in.
When you attempt shadow sneak to get out of the boundary while look forward, you would just teleport along the wall of the boundary. Instead, you have to look down and zoom out, then select a place outside the boundary to teleport to. Looking backwards may help.
If a player uses Vehement Blizzard on the boundaries, the snow after the ultimate finishes will show the boundaries.