I know that any compact orientable 3-manifold can be obtained from the three sphere $S^3$ by an integer surgery. I am not sure why the surgery operation is completely determined by Where we map meridians of the tori. More specifically,

an integer surgery is determined by specifying a link in $S^3$ and a framing for each component of that link  (or equivalently an integer for each component). For the sake of simplicity in this question I will assume that my link has a single component (a knot).

To perform the surgery we cut out a regular neighborhood of the knot (which is a solid torus) and then we glue back a solid torus by a certain homeomorphism between the surfaces of these two solid tori. What is not clear to me is that why this homeomophism is determined by declaring where the homeomophism maps meridian? (in the case of the integer surgery we map the meridian to the framing of the knot). In other words, why the map between these two curves on the tori determine the entire homeomorphism (and hence the surgery). If I am missing something I would also appreciate a correction. I know this is not a research question so if this is  not the right place to ask this question I would appreciate a pointer where to post this question.