What is a truckle bed (from Romeo and Juliet)?

If you ever have overnight guests, a truckle bed might come in handy. This is a low, space-saving bed on small wheels (or casters) that can be rolled under another bed when not in use. You might know it by another name: trundle bed.

In Romeo and Juliet, Mercutio calls out to Romeo, who is hiding in an orchard:

Romeo, good night. — I'll to my truckle-bed;
This field-bed is too cold for me to sleep . . .