All it says is that after gaining an electron, #"F"# was stabilized by less compared to how much #"Cl"# was stabilized.
First of all, let's make sure we set this straight... electron affinity is negative. So #|EA|_F < |EA|_(Cl)# is really what you're saying, and #EA_F > EA_(Cl)#. That is, #"F"# becomes less stabilized than #"Cl"# does when it gains an electron.
And that isn't necessarily intuitive. As it turns out, the atomic radius of #"F"# is so small compared to that of #"Cl"# that adding an electron into #"F"# to form #"F"^(-)# is not as easy.
The small electron-dense #"F"# atom has a harder time getting that electron to stay, so the electron affinity of #"F"# is less negative.
That is, #"F"^(-)# is less stabilized relative to #"F"# than #"Cl"^(-)# relative to #"Cl"#.
