Max Load of a material determines how much load (Mass) every side of it can hold. You can obtain this information using the Item Stats functionality added to the game in the alpha 7.6. To do this, place the mouse over an item to see the Object Stats, including its Max Load.
Structural Integrity = RoundDown( Max Load ÷ Mass )
The function of Max Load is easily explained using an example.
Brick Blocks have a Max Load of 65 and a Mass of 13. Each side of the block may hold up to 65 mass (65 ÷ 13 = 5). This means the player may connect five Bricks or other blocks to a mass of less than 65, to every side of a supported Brick block before the construction breaks. If you add a sixth block to one of the beams the whole beam collapses. Even if you make turns in your construction, if the Max Load for a side is reached the block breaks.
Video Tutorial (Alpha 13)
The video below explains the physics of mass and max load for various building materials in detail as of alpha 13.