It is pronounced "be-STOW", so the stressed syllable is "-stow".
A stressed syllable is the part of a multi-syllabic word that you put emphasis on when you're saying it.
Ex.
-"rive" in "ar-rive"
-"junc" in "con-junc-tion"
- "med" in "im-med-i-ate"
If you still don't understand it, try saying it out loud dramatically and you'll hear which part is slightly "louder", or more stressed :)
Footprint is stressed on the first syllable.
Behave is stressed on the second syllable.
The first syllable is stressed.
No, allow is stressed on the second syllable.
Saunter is stressed on the first syllable.
District is stressed on the first syllable.
The first.
The first.
The first.
In the word "lawyer," the first syllable "law" is stressed.
Complain is stressed on the second syllable.
August is stressed on the second syllable.