Thread: string/yarn used to stitch fabric together. It is usually a polyester or cotton-wrapped polyester string.
It is important to use good quality thread that fits the project you are doing (ie: heavier thread for denim topstitching, very fine thread for stitching with lightweight silk or lawn.) Also, match colors! If you cannot find an exact match, going with a shade darker will be more subtle because seam lines have shadows.
Never buy off-brand cheap thread (you know the kind- like $1 for a spool). Also, do not use vintage thread. Both are weak, will break and be frustrating to sew with. Don't sew seams with machine embroidery thread.
Brands I like? I stick to buying Gutermann (very nice) and Dual Duty threads.