OK.  I did it.  I drew a triangle, then measured the length of each side.  
The lengths of the sides are 3 inches, 4 inches, and 5 inches. 
(7.62 cm,  10.16 cm,  and  12.7 cm) 
"Perimeter" means the distance all the way around it, so I added them up.  
I found that the perimeter of my triangle is 12 inches. (30.48 cm)
__________________________________
Now I have a question for YOU:
Why couldn't YOU do that ?
Take a pencil, paper, and ruler, draw a triangle on the paper, 
then measure the lengths of the sides and add them up ?
Which part of that didn't you understand ?