This example fits a GP and plots something. I am trying to understand all the objects here.

```
using Stheno
using Stheno.AbstractGPs
using CairoMakie
# Short length-scale and small variance.
const l1 = 0.4
const s1 = 0.2
# Long length-scale and larger variance.
const l2 = 5.0
const s2 = 1.0
# Specify a GaussianProcessProbabilisticProgramme object, which is itself a GP
# built from other GPs.
f = let
gpc = Stheno.GPC()
gp = kernel -> Stheno.wrap(GP(kernel), gpc)
f1 = s1 * stretch(gp(Matern52Kernel()), 1 / l1)
f2 = s2 * stretch(gp(SEKernel()), 1 / l2)
f3 = f1 + f2
Stheno.GPPP((; f1, f2, f3), gpc)
end
# Generate a sample from f3, one of the processes in f, at some random input locations.
# Add some iid observation noise, with zero-mean and variance 0.05.
const n = 1_000
const xx = collect(range(-5.0, 5.0; length=n))
const x = GPPPInput(:f3, xx);
const σ²_n = 0.05;
const fx = f(x, σ²_n);
const y = rand(n) + sin.(xx .* 2);
f_posterior = @time posterior(fx, y);
x_plot = range(-7.0, 7.0; length=1000);
xp = GPPPInput(:f3, x_plot);
ms = marginals(f_posterior(xp));
mea = mean.(ms)
st3 = 3std.(ms)
fig = Figure()
Axis(fig[1, 1])
scatter!(x.x, y; color=:red, markersize=2, strokewidth=0);
lines!(x_plot, mean.(ms), color = :blue, linewidth = 2)
band!(x_plot, mea .- st3, mea .+ st3, color = (:blue, 0.2))
for col in eachcol(rand(f_posterior(xp), 10))
lines!(x_plot, col, color = (:blue, 0.3))
end
fig
```

What does this part do?

```
const x = GPPPInput(:f3, xx);
const σ²_n = .1;
const fx = f(x, σ²_n);
```

Especially `fx = f(x, σ²_n);`

. What do the parameters of `f`

mean?