Arabic words are often composed of 3-letter or 4-letter root words. These roots have meanings of their own; but other letters can be added to them to open up a myriad of new possibilities that maintain the root's essential meaning.

For example, خد (khadd) means "cheek"
Ma-Khad-da literally means "that on which you place your cheek (i.e. pillow)

There are standardized formulas for such addition. For example, and similar to the example above:

‎سطر (ṣatr) means "line"
‎مسطرة (maṣtarah) means the tool with which you draw a line (i.e. a ruler)

Note: the Spanish word for pillow is “almohada” which is borrowed from the Andalusian Arabic almukhadda from classical Arabic almikhadda