The third of the four இலக்கண வகை types: an இடைச்சொல் carries no independent meaning by itself - it serves between a noun and a verb, in one of 4 forms.

Remove every இடைச்சொல் from a paragraph and the individual words still make sense, but the connections between them vanish entirely.

மரமும் பூவும் - "உம்" here is an இடைச்சொல் meaning "and".