The electrophile involved in this reaction is:

In the Reimer-Tiemann reaction, $\text{NaOH}$ reacts with ${\text{CHCl}}_3$ to generate the reactive intermediate dichlorocarbene via alpha-elimination.
$${\text{CHCl}}_3+{\text{OH}}^{-}\to {\text{CCl}}_3^{-}\to :{\text{CCl}}_2+{\text{Cl}}^{-}$$
The neutral carbene acts as an electrophile and attacks the phenoxide ring.