Toile or muslin is a test garment that you should make while trying out patterns you draft yourself or new patterns from company you never used before or learning new techniques, sewing more complicated pieces like jackets or coats. Making a toile from cheap fabric allows you to alter pattern to your figure without wasting your fashion fabric.
Toile fabric should be inexpensive, plain woven fabric or in some cases, if you want to try out a pattern with certain type of fabric - one that is similar in drape and weight to fabric you want. You'll burn through so it's the best to use up cheap cotton, clearance sale fabrics or even old bed sheets.
Depending on your skill level you might just sew a toile for fitting without finishing it like real garment, but while learning to sew more complicated garments like blazers or coats, you should try to finish it off like a real piece. Toiles can be finished to be wearable around home, you'll learn proper finishing techniques and be more precise with your sewing without wasting more expensive, nicer fashion fabric.
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