If your arrow is too stiff in spine and you are shooting right handed, your arrow will come out of the bow with the nock end too far to the right. This will cause a fletched arrow to fish tail as it tries to straighten itself. One way to correct this is increase the weight of your field point.
If your arrow is too weak in spine and you are a right hand shooter, the arrow will leave the rest nock left. Again your arrow will fish tail and fly poorly as it tries to correct or straighten itself in flight. You can correct this by decreasing the arrow point weight or shortening the shaft.
If you are shooting left handed the reverse will take place. Too stiff-nock left; too weak-nock right.
Determining correct arrow spine with a fletched shaft can be difficult. The arrow may be quicker than the eye and you can't determine nock left from nock right. Find an article on bare shaft tuning. There may be a good one in the Shooters Forum on this site.
If you are new to shooting and still working on your form, bare shaft tuning can be frustrating. Each time you change you form, even in the slightest way, the bare shaft will react differently. Nail down your form first so each release is the same. Makes tuning much less frustrating. Good Luck.