This is a fairly common question, and can be tackled from different angles. Here is my take on it.
On the highest level, meditation is our true nature; therefore, it is effortless. However, our habitual mental patterns cast a cloud of noise above it, and we don’t perceive nor live that reality. In this sense, we call meditation the technique(s) that we use to free ourselves from these patterns – to see through them and reach a deeper level in our being/consciousness. That requires effort.