Tie dye is a term used to describe the tie-resist technique of making patterns on fabric. Tie-dye includes various methods (for example Bandhani is a type of tie-dyeing), but the basic principle is that the fabric is tied in some manner and then dyed. The areas where the fabric has been tied (tightly) doesn’t allow the dye to percolate through so you finally end up with a fabric that has patterns where the fabric was not dyed.
Tie-dyeing techniques include leheria from Rajasthan (parallel lines tied after rolling the fabric diagonally), bandhani from Gujarat (tying the fabric in tiny circles before dyeing), and also methods where the fabric is crushed or twisted before tying it at random intervals.
Fabrics can also be dyed before tie-dyeing if you want two colours on the fabric.