Look up the standard yarn-weight category, ply and typical gauge.
Yarn is grouped into weight categories that describe how thick the strand is, from lace at the thinnest to jumbo at the thickest. The Craft Yarn Council numbers them from 0 to 7, with lace as 0, fingering as 1, sport as 2, DK as 3, worsted as 4, bulky as 5, super bulky as 6, and jumbo as 7. Each category has a typical gauge and a recommended needle or hook range.
Names are not always consistent between countries, since the same name can mean different yarn in the US and in the UK. Ply, the number of strands twisted together, does not reliably show thickness, because plies can be thin or thick.
When you replace the yarn in a pattern, match the gauge and not just the name. Knit or crochet a swatch with the new yarn and compare it with the gauge in the pattern, and check that the fiber suits the project, since cotton, wool, and acrylic hang and wear differently.
Use the ball band as your guide, since it gives the weight category, the recommended needle size, and the gauge. A different yarn with the same category and gauge usually works, but the amount you need may differ, so buy by yardage.