the literal meaning of sawm is to restrain oneself from something. for example, allah tells us in al-quran that maryam said,

"surely i have vowed to ar-rahman (the most merciful, allah) to fast (sawm)."

[noble quran 19:26]

the fast here means to be silent, that is, to abstain from speaking. islamically, sawm means to abstain from food, drink and sexual relation between dawn and sunset.