Question: "what is the geometric meaning of the sections in $H^0(T_X(d))$? Are they a sort of vector fields with special properties?"
Answer: Let $C:=\mathbb{P}^1:=\mathbb{P}(V^*)$ with $V:=k\{e_0,e_1\}$. If you want to relate the invertible sheaf $L:=\mathcal{O}_{C}(d)$ ($d \geq 0$) to geometry, maybe a good idea is to consider the $d$-uple embedding
$$\phi_d: C \rightarrow \mathbb{P}^d:=X.$$
It follows $\phi_d^*(L) \cong \mathcal{O}_{C}(d)$. The message has been deletedtangent sheaf $\mathcal{T}_C \cong \mathcal{O}_{C}(2)$ on $C$ hence you are interested in the tensor product
$$T_C(d):=T_C \otimes \phi_d^*(L)$$
and the author hasglobal sections
$$H^0(C, T_C(d)) \cong H^0(C, \mathcal{O}_C(d+2)) \cong Sym^{d+2}(V^*)$$
where $V^*:=k\{x_0,x_1\}$. Maybe you can relate these global sections to the embedding $\phi_d$. There is a well defined multiplication map
$$d: Sym^2(V^*)\otimes Sym^d(V^*) \rightarrow Sym^{d+2}(V^*)$$
with $d(u\otimes v):=uv$.
If $I_d \subseteq \mathcal{O}_{X}$ is the ideal sheaf of $C$ in the $d$-uple embedding there is an exact sequence
$$C1.\text{ }0 \rightarrow I_d/I_d^2 \rightarrow \phi_d^*(\Omega^1_{X/k}) \rightarrow \Omega^1_{C/k} \rightarrow 0,$$
hence the cotangent sheaf $\Omega^1_{C/k}\cong \mathcal{O}(-2)$ may be constructed as a quotient of the pull back of the cotangent sheaf of $X$. This is the dual of the tangent mapping wrto $\phi_d$. If you want to relate invertible sheaves and tangent bundles/cotangent bundles to "geometry" you must study the relationship between invertible sheaves and maps to projective space and the sequence $C1$. Tensor the sequence $C1$ and dualize to get the sequence
$$ 0 \rightarrow T_C \otimes \mathcal{O}(d) \rightarrow \phi_d^*T_X \otimes \mathcal{O}(d) \rightarrow Hom(I_d/I_d^2\otimes \mathcal{O}(-d), \mathcal{O}) \rightarrow 0$$
and take global sections to get
$$G1.\text{ } 0\rightarrow H^0(C, T_C \otimes \mathcal{O}(d)) \rightarrow H^0(C, \phi_d^*T_X \otimes \mathcal{O}(d)) \rightarrow H^0(C, Hom(I_d/I_d^2\otimes \mathcal{O}(-d) , \mathcal{O}))\rightarrow \cdots $$
The above sequence relates the global sections of $T_C\otimes \mathcal{O}(d)$ to the embedding $\phi_d$. You should identify the image of the left map as a subspace of the forumvector space of global sections of $\phi_d^*T_X \otimes \mathcal{O}(d)$. Locally for maps of rings $k \rightarrow A \rightarrow B$ there is the tangent sequence
$$0 \rightarrow Der_A(B) \rightarrow Der_k(B) \rightarrow B\otimes_A Der_k(A),$$
and when $B:=A/I$ you get the sequence
$$R1.\text{ }0 \rightarrow Der_k(A/I) \rightarrow A/I \otimes_A Der_k(A) \rightarrow Hom_{A/I}(I/I^2, A/I) \rightarrow \cdots .$$
If $Y:=Spec(A/I) \subseteq X:=Spec(A)$ it follows the sequence $R1$ relates vector fields on $Y$ to vector fields on $X$ "parallel" to $Y \subseteq X$. The module $A/I \otimes_A Der_k(A)$ is the restriction of $Der_k(A)$ to the closed subscheme $Y:=V(I)\cong Spec(A/I)$.
Note: In the case of the projective line, when you pull back the cotangent bundle you get a decomposition
$$\phi_d^*(\Omega^1_{X/k}) \cong \oplus_i \mathcal{O}_C(d_i)$$
into direct sums of invertible sheaves. There is the notion k-very ample line bundle that may be of interest. Note also that when you have a closed embedding $\phi_d:\mathbb{P}^1 \subseteq \mathbb{P}^d$, you may consider global sections of $T_{\mathbb{P}^d}$ - global vector fields $\partial$ on $\mathbb{P}^d$. The global vector fields $\partial$ that are parallel to $\mathbb{P}^1$ in the embedding $\phi_d$, restrict to vector fields on $\mathbb{P}^1$. You should relate this to the sequence $G1$.
