Their angles are equal. Their sides are proportional. For example, if a smaller triangle has the angles 30, 60 and 90 degrees, then the larger similar triangle has the same degrees. If a smaller triangle has sides 3, 4 and 5, then the larger similar triangle could have sides 6, 8 and 10 or 9, 12 and 15. Etc., etc....
I hope this helps.