Exactly What It Says On The Tin.
Let's break it down by looking at the definition. An LCG is any PRNG that takes the form
where $x_0,a$ and $b$ are some integer parameters, and $M$ is a large integer only slightly below the largest representable integer on the machine.
We can see where the name comes from (which, BTW, is due to D.H. Lehmer): "linear" is due to the fact that the quantity whose remainder we are taking is the result of a linear function ($ax+b$), and "congruential", since we are performing a congruence operation (modulo).