I need help with Battle Master's saving throws

What are they rolling against when they have to make a saving throw on the move i used for example.  Trip Attack:  When you hit a creature with a weapon attack, you can expend one superiority die to attempt to knock the target down. You add the superiority die to the attack's damage roll, and if the target is Large or smaller, it must make a Strength saving throw or be knocked prone.  

Saving Throws .  Some of your  Maneuvers  require your target to make a saving throw to resist the maneuver’s effects. The saving throw DC is calculated as follows: Maneuver save DC  = 8 + your proficiency bonus + your  Strength  or  Dexterity  modifier (your choice)

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