Simple Linear Model Interpretation

Why is the model called simple?

It is called simple because there is only one predictor, $X$.

When there is more than one predictor, say $k$ predictors: $X_1,X_2,\dots ,X_k$, then the model is called multiple. See The Multiple Regression Model.

Interpretation of ${\beta }_1$

When $x_i$ is increased by 1 unit, there is a change (increase or decrease depends on the sign) of ${\beta }_1$ unit in $y_i$.