There are two ways to think of e : σ. One is "the expression e has type σ", another is "the ordered pair of the expression e and the type σ".
View Γ as the knowledge about the types of expressions, implemented as a set of pairs of expression and type, e : σ.
The turnstile ⊢ means that from the knowledge on the left, we can deduce what's on the right.
The first rule [Var] can thus be read:
If our knowledge Γ contains the pair e : σ, then we can deduce from Γ that e has type σ.
The second rule [App] can be read:
If we from Γ can deduce that e_0 has the type τ → τ', and we from Γ can deduce that e_1 has the type τ, then we from Γ can deduce that e_0 e_1 has the type τ'.
It's common to write Γ, e : σ instead of Γ ∪ {e : σ}.
The third rule [Abs] can thus be read:
If we from Γ extended with x : τ can deduce that e has type τ', then we from Γ can deduce that λx.e has the type τ → τ'.
The fourth rule [Let] is left as an exercise. :-)
The fifth rule [Inst] can be read:
If we from Γ can deduce that e has type σ', and σ' is a subtype of σ, then we from Γ can deduce that e has type σ.
The sixth and last rule [Gen] can be read:
If we from Γ can deduce that e has type σ, and α is not a free type variable in any of the types in Γ, then we from Γ can deduce that e has type ∀α σ.