Word stress indicates which syllables are stressed - or emphasised - in a word.

Example
‘Photograph' has word stress Ooo (stress on the first syllable), ‘photographer' has word stress oOoo (stress on the second syllable).

In the classroom
There are many ways to indicate word stress visually, including the method above, putting a line above main stress and below secondary stress, and using a board marker to partially rub out unstressed syllables.