Each picture shows 3 similar triangles.
They are called similar because they are the same shape, they have the same angles and the lengths of the edges are in the same ratio. For example in this diagram the edges in each triangle are all in the ratio 2 : 3 : 4 and (from left to right) the ratio of the edge lengths in the 3 triangles is 2 : 4 : 6 or equivalently 1 : 2 : 3. The triangles are enlargements of each other. The middle triangle is an enlargement of the triangle on the left by a scale factor 2, and the triangle on the right is an enlargement of the triangle on the left by a scale factor 3.