Isosceles means having two sides of equal length, because a triangle with two out of its three sides the same length is an isosceles triangle.
Triangles have three sides (tri=3) and three angles. In an equilateral triangle, all sides are the same length. But in an isosceles triangle, two sides are the same length and the third is not.
You can see an example from the picture. One cute way to remember the name of the triangle with two equal sides is to remember that it sounds like “eyes” in the name. You have two eyes, and an isosceles triangle has two equal sides.