To picture what's happening in the boom, imagine that it was too weak to support the load - how would it fail? It's pretty obvious that it would collapse due to compressive forces (as opposed to being pulled apart by tension). So theh "thrust along the boom" means the compressive forces in the boom. These compressive forces are generated by (a) the tension in the cable pressing down on the boom at the top, and (b) the supporting force from the ground holding the bottom of the boom up. Thee two forces must be equal and opposite.
To solve this type of problem you need to set up a free body diagram that shows the horizontal and vertical components of force at the top of the boom and at the bottom. Then solve such that the sum of forces in the horizontal direction is zero, and the sum of forces in the vertical direction is zero.