Struggling is to attempt to free oneself from an unwanted grip or position. A person can struggle in their bonds or hold. A dictionary definition of "struggle" is to strive, to labour in difficulty, to fight (for or against), to contend.
In spanking and BDSM, struggling can be unwelcome behaviour as it is unsubmissive. But pretended struggling, or struggling in vain, can also be a welcome element that adds to the eroticism of a scene, making the bondage and/or helplessness believable and adding to the power exchange. See also agonophilia.