To untie means to release or loosen a knot or tied thing.
When we untie something, it's often because it was holding us back or making our lives more complicated. Imagine wearing shoes with laces that are too tight - if you can't walk properly, you might want to untie them and take off the shoes for a bit. Or picture a bag full of clothes that's tied up with a string so tightly that you can barely carry it - you'd probably need to untie it to get everything out safely.
(transitive) To loosen, as something interlaced or knotted; to disengage the parts of.
Example: to untie a knot
