A taproot is an enlarged, somewhat straight to tapering plant root

Root

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...

 that grows vertically downward. It forms a center from which other roots sprout laterally.
Plants with taproots are difficult to transplant

Transplanting

For botanical organ transplant, see GraftingIn agriculture and gardening, transplanting or replanting is the technique of moving a plant from one location to another. Most often this takes the form of starting a plant from seed in optimal conditions, such as in a greenhouse or protected nursery...

. The presence of a taproot is why dandelions are hard to uproot—the top is pulled, but the long taproot stays in the ground, and resprouts.

The taproot system contrasts with the fibrous root system

Fibrous root system

A fibrous root system is the opposite of a taproot system. It is usually formed by thin, moderately branching roots growing from the stem...

 with many branched roots.
Many plants (the entire clade of dicots, one of the two divisions of angiosperms) start with a taproot, which is one main root forming from the enlarging radicle of the seed.