To constrain is to hold back, restrain, or confine. If you've ever had to constrain a toddler in the middle of a temper tantrum, you know you have to watch out for flailing limbs.
The verb constrain comes from the Latin word constringere, which means to bind together or tie tightly. Constrain can imply both literal and figurative bindings. You can constrain someone by physically holding on to something like a person's shirt. Or, you can figuratively constrain someone by putting up obstacles. Your teenagers, for example, may feel constrained by your rules and accuse you of sucking the fun out of their social lives.