If $a$ is a scalar and $M$ is a square matrix, it is very convenient to be able to write $a + M$. Usually people know immediately what this means, but are uneasy about “abusing” notation, so here’s the detailed justification for why this is perfectly legitimate:

Matrices should be considered first and foremost as linear transformations. You know what a matrix is if you know what it does to vectors. A scalar is also a linear transform on vectors: multiplying a scalar times a vector is a linear operation. Therefore, scalars can also be thought of as linear transformations, and therefore as matrices. It is immediate which matrix the scalar should be: the result of multiplying by a scalar $k$ is that all components are scaled by $k$; the matrix that does this is just $kI$.