"sedang" is a word that is used as a time marker to indicate that the action is still ongoing.
A sentence like that should be translated in the "present continuous tense" in English.
"Dia tidur" = He/She sleeps/slept.
"Dia sedang tidur" = He/She is/was sleeping.
"sedang" can be replaced with the word "lagi".
Tono sedang makan = Tono lagi makan = Tono is eating (action is still ongoing).
But remember that when using "lagi" for this purpose, "lagi" is positioned in the same place as "sedang", right before the verb, as part of the predicate.
If you put "lagi" after the predicate, it has a different meaning:
Tono lagi makan = Tono is eating.
Tono makan lagi = Tono eats again.