You can see the shader parameters by expanding the Shader Inputs at the top of the shader code at shadertoy. In your case they would be
uniform vec3 iResolution; // viewport resolution (in pixels)
uniform float iGlobalTime; // shader playback time (in seconds)
uniform float iTimeDelta; // render time (in seconds)
uniform int iFrame; // shader playback frame
uniform float iChannelTime[4]; // channel playback time (in seconds)
uniform vec3 iChannelResolution[4]; // channel resolution (in pixels)
uniform vec4 iMouse; // mouse pixel coords. xy: current (if MLB down), zw: click
uniform samplerXX iChannel0..3; // input channel. XX = 2D/Cube
uniform vec4 iDate; // (year, month, day, time in seconds)
uniform float iSampleRate; // sound sample rate (i.e., 44100)
Most of those inputs are redundant unless the shader is animated. ShaderToy also has support for playing audio so you can get rid of those aswell. Keep in mind that the code there is fragment shader code, you'll probably get away with the default vertex shader found in the SFML docs in most cases I think.
I don't personally like ShaderToy for these reasons, it makes learning glsl kinda hard because of the extra stuff the code tends to have. It's good for demonstration, but I'm usually just googling for general ideas of what I want and go from there.
The result of this shader seems fairly simple, basically it just looks like a horizontal blur applied after rendering the screen with horizontal and vertical lines (which you also do in the shader) and some gamma adjustments.
GLSL is essentially just C with some extra keywords and functions, it's not so intimidating once you have tossed off even the most basic shader, like tint effect or texture overlay for example
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