The first order MA filter is a causal filter which has a two samples (finite) impulse response (FIR). In the presented example, the first term is taken to be equal to 0.5. It is possible for you to study the behaviour of the filter in function of its second coefficient (b) which can be varied between -0.5 and 0.5. If b is negative, the filter is a high-pass filter with maximum attenuation at the null frequency. If b is positive, the filter is a low-pass filter with maximum attenuation at the Nyquist frequency. First order MA (MA1) filters are relatively inefficient. The example shows the impulse response (in time), the modulus of the FT with a linear scale, the modulus of the FT in dB and the phase of the FT.