A triangle has an interior angle total of 180 degrees, and if any of those three angles are obtuse, that forms an obtuse triangle.
Now suppose we had a triangle with two acute angles, but not having those two angles add up to more than 90 (consider 30 degrees and 20 degrees).
We can find the unknown angle through some quick subtraction (180 - 30 - 20 = 130).
Since one of the angles is an obtuse angle, this triangle is an obtuse triangle, even though this triangle possesses two acute angles (try drawing a picture, it'll help you visualize it), meaning that a triangle with two acute angles will only sometimes be an acute angle (when those two angles add up to an angle measure more than 90, meaning that the last angle will also be less than 90, making an acute triangle).
Hope this helps!