Hilo vs. hebra
While both hilo and hebra can be translated to English as thread, they are used differently in the context of sewing. The word hilo is the general term for the continuous material or the thread wound around a spool. In contrast, the word hebra refers to a single, individual strand, a fiber, or a specific cut piece of hilo that is ready to be passed through the eye of a needle.
Hilo
The word hilo refers to the continuous material of thread in general, often bought, measured, and stored on a spool or reel.
Necesito comprar un carrete de hilo negro para coser este botón.
(I need to buy a spool of black thread to sew this button.)
El sastre utiliza hilo de algodón de alta calidad para sus trajes.
(The tailor uses high-quality cotton thread for his suits.)
Se me acabó el hilo rojo a mitad de la costura.
(I ran out of red thread halfway through the sewing.)
Perdí el hilo de la conversación porque me distraje con el ruido.
(I lost the thread of the conversation because I got distracted by the noise.)
Este vestido tradicional está bordado con hilo de seda muy fino.
(This traditional dress is embroidered with very fine silk thread.)
Hebra
The word hebra refers to a single, detached strand, a distinct fiber, or a cut piece of hilo used for a specific sewing task.
Corta una hebra larga para que no tengas que enhebrar la aguja dos veces.
(Cut a long strand of thread so you do not have to thread the needle twice.)
Tira de esa hebra suelta para deshacer el dobladillo del pantalón.
(Pull that loose strand of thread to undo the hem of the pants.)
Pasó la hebra por el ojo de la aguja al primer intento.
(She passed the cut piece of thread through the eye of the needle on the first try.)
Para esta receta tradicional necesitas agregar unas pocas hebras de azafrán.
(For this traditional recipe you need to add a few strands of saffron.)
El gato estaba jugando con una hebra que cayó de la mesa de costura.
(The cat was playing with a strand of thread that fell from the sewing table.)
Summary
In short, use hilo when talking about the continuous thread material itself or the spool, and use hebra when referring to a single, cut strand or fiber of that material.